An unknown cell subpopulation was observed in mouse and rat thymus, spleen and bone marrow cells, as well as in human peripheral blood mononuclear cells (resting and stimulated by PHA) using equilibrium HCl/acridine orange staining. This subpopulation includes cells with decreased green and unchanged red fluorescence. The staining does not affect cells in S- and G2/M-phases. The mechanism and biological meaning of the effect await further investigation.