3-amino-6-methoxy-9-(2-hydroxyethylamine) acridine (AMHA) is an acridine derivative, which is easily excited in near ultraviolet and which emits a bright green fluorescence. The dye was preferentially incorporated into nucleic structures as attested by microscopic and cytometric analyses after RNase and DNase treatments. The affinity for RNA seemed low and similar to that observed for propidium iodide. AMHA was quickly accumulated in fixed cells, and in appropriate concentrations (10-50 microM) was a DNA- and RNA-specific dye. AMHA probably exhibits an adenine-thymine specificity, as suggested by its quenching after bromodeoxyuridine uptake: the fluorescence quenching was similar to that obtained for Hoechst 33258. After cell treatment by RNase and in the presence of MgCl2, AMHA staining allowed flow cytometric analysis of the cell-cycle distribution. The resulting histograms were similar to those obtained with propidium iodide (CV near 3.5%, and similar cell cycle distribution). Thus, AMHA is a suitable fluorescent dye for efficient analysis of the cell cycle by flow cytometry.