Mouse bone marrow cells were separated according to their light scattering properties in a light-activated cell sorter. The intensity of the scattered light was measured in a forward direction (FLS) and perpendicular to the incident light (PLS). These two parameters were found to give an indication of cell size and shape. The fractions containing cells with different light scattering properties were assayed for CFU-s (pluripotent stem cells) content and for the incidence of three types of CFU-c (cells committed to granulocyte/macrophage differentiation). These measurements confirm previous size calculations based on sedimentation rate and density separation. During maturation, the different colony-forming units (CFU) show a gradual increase in PLS intensity. This suggests that the transition from CFU-s through to CFU-c 3 is accompanied by a morphological differentiation.