The studies described here explored the staining of acute leukemia cells with acridine orange (AO). The red fluorescence curve of AML specimens was usually bimodal, suggesting the presence of subpopulations of cells which have different RNA contents. In almost every AML specimen, small leukemic blast cells comprised at least part of the "low RNA content" subpopulation. Residual granulocytes and lymphocytes also contributed to this population. Frequently, the green fluorescence, indicative of the binding of AO to DNA, was slightly less in these cells than in the majority of cells present. There was no evidence however, that the leukemia cells with these characteristics represented a G0 or kinetically quiescent population of cells. In ALL specimens, the presence of multiple cytogenetically distinct clones was easily detectable in AO stained specimens. The red fluorescence curve of G0/G1 ALL cells was unimodal.