Fluorescein derivatives are the most common fluorescent reagents for biological research because of their high absorptivity, excellent fluorescence quantum yield, and good water solubility.

Fluorescein-based dyes and their conjugates have several performance characteristics that may facilitate or limit use in certain applications. Fluorescein displays:

  • A relatively high rate of photobleaching
  • Fluorescent signal that is sensitive to pH changes
  • A relatively broad fluorescence emission spectrum
  • Fluorescence quenching on conjugation to biopolymers
We offer a broad range of fluorescein conjugates and derivatives as well as a series of high-performance fluorophores with improved characteristics for labeling and detection.

Fluorescein dashboard

Initial brightness



Fluorescein is a bright green fluorphore that is sensitive to changes in pH and can photobleach rapidly unless an antifade agent is used. Anti-fluorescein antibodies can be useful in amplifying a weak signal.


Photostability in buffer

Photostability in antifade



Fluorescein products

We offer a broad range of fluorescein products including secondary antibodies, primary antibodies, bioconjugates, and more

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HeLa cells were fixed and permeabilized using the Image-iT® Fixation/Permeabilization Kit then incubated with Anti- ATP Synthase Subunit IF1 and GAM-FITC secondary Ab. Cells were counterstained with NucBlue® Live and ActinRed 555 then mounted using ProLong® Gold Antifade Reagent and imaged using the EVOS® FL Auto Imaging System and a 40X CoverSlip corrected Objective

Fluorescein reactive dyes

Select dye-labeling chemistries for conjugation with the following reactive groups: 

Amine Thiol Aldehyde, ketone, carboxylate, or phosphate

Fluorescein protein/antibody labeling kits

Select an optimized labeling kit for your target antibody or protein:

Target protein Kit Cat. No.
Site-specific antibody labeling
<1–20 µg mouse IgG1 Zenon® Fluorescein Mouse IgG1 Labeling Kit Z25042
<1–20 µg rabbit IgG Zenon® Fluorescein Rabbit IgG Labeling Kit Z25342
Conventional antibody/protein labeling
1 mg antibody/protein Fluorescein Antibody/Protein Labeling Kit F10240

High-performance alternatives to fluorescein

Fluorescent conjugates

The Alexa Fluor® 488 dye—with spectral properties and quantum yield nearly identical to those of fluorescein isothiocyanate (FITC)—produces brighter, more photostable conjugates. These conjugates are ideal for imaging and other applications requiring increased sensitivity and environmentally insensitive fluorescence detection.

Monitoring pH

For pH monitoring the pHrodo™ green dye is almost nonfluorescent at neutral pH and fluoresces brightly in acidic environments. Unlike fluorescein, the minimal dye fluorescence at neutral pH also eliminates the need for wash steps and quencher dyes because any non-internalized dye will be essentially nonfluorescent.

  Comparison of the relative fluorescence of goat anti–mouse IgG antibody conjugates prepared from Alexa Fluor® 488 dye and from fluorescein isothiocyanate (FITC).
Conjugate fluorescence is determined by measuring the fluorescence quantum yield of the conjugated dye relative to that of a reference dye and multiplying by the dye:protein labeling ratio.



Molecular Probes® Handbook

Molecular Probes HandbookThe significantly revised 11th Edition is the most complete fluorescence labeling and detection reference available. The Molecular Probes® Handbook can help you achieve the best experimental results for your fluorescence labeling and detection experiments. View the handbook online or order your own copy.


Antifade reagents

Antifade reagentsAntifade reagents suppress photobleaching and preserve the signals of your fluorescently labeled target molecules. They offer excellent compatibility with fluorescein dye and a multitude of dyes across the spectrum, making them especially valuable tools for multicolor applications.

Cell stain tool

Cell stain toolStain your own cell by using our simulation tool to develop reproducible results with many of our signature fluorescent dyes. Develop your multiplexing strategy for antibody probes, reactive dyes, and counterstains to create your perfectly labeled fluorescent cell.


Fluorescence SpectraViewer

Fluorescence SpectraviewerPlot and compare spectra and check the spectral compatibility for common fluorophores including all the Molecular Probes® fluorescent probes. The Fluorescence SpectraViewer tool is formatted for both microscope imaging and flow cytometry applications.