ACMA (9-amino-6-chloro-2-methoxyacridine) is a DNA intercalator that selective binds to poly (d(A-T)). Excitation of the ACMA-DNA complex (excitation/emission maxima ∼419/483 nm) is possible with most UV-light sources, making it compatible for use with both shorter- and longer-wavelength dyes. ACMA also apparently binds to membranes in the energized state and becomes quenched if a pH gradient forms. It has been extensively employed to follow cation and anion movement across membranes and to study the proton-pumping activity of various membrane-bound ATPases.