A correlation between cell size and cycle state is demonstrated for mitogen stimulated murine lymphocytes. Cell cycle analysis was performed using multiparameter cytofluorometric analysis of acridine orange stained cells. Cell size was determined by measuring "time of flight" of axial light extinction. Cells with diameters of 5.0 - 6.0 micron are shown to be predominately in G0. Among stimulated cells, those 6.0 - 7.0 micron in diameter are predominately early and late G1 (80%) with some S phase cells. Cells in G2 + M were found to predominate among lymphocytes with diameters of 7.5 - 12 micron. This approach provides a method for isolation of lymphocytes in specific phases of the cell cycle which are suitable for subsequent functional studies.