Mouse effector cells mediating broadly reactive anti-tumor cytotoxic activity, induced under syngeneic conditions in vitro with polyinosinic acid and phorbol-induced lymphocyte growth factors, can be generated not only by unprimed spleen, thymus, and fetal liver cells, but also (unlike classic cytotoxic T lymphocytes) at augmented levels by spleen cells depleted of Thy 1-bearing cells and thymocytes depleted of Lyt 2-bearing cells. Higher levels of H-2 expression on mitogen-induced mouse lymphoblasts in the G2 than G0 or G1 portions of the cell cycle have been demonstrated by flow microfluorometry. This difference cannot be attributed to differences in cell size determined by low angle light scatter. The cholesterol probe, filipin, has been adapted to flow microfluorometry for quantitative detection of cholesterol at the single cell level.