T, B, and null lymphocytes isolated by sheep red blood cells (SRBC) rosetting or adherence to a nylon wool column had distinct size differences according to Coulter counter size analysis. The diameters in microns were as follows: B = 6.42, T = 6.76, null = 7.16, granulocytes = 8.42, and monocytes = 9.02. Coulter size distributions of cell preparations were compared to the proportions of cells with E, EAC, and surface immunoglobulin (SIg) markers in these preparations as well as their reactivity to HLA--DR antisera and rabbit anti-B-lymphocyte antisera. Cell preparations monitored by Coulter sizing produced the expected results in mixed lymphocyte culture (MLC) testing. That is, T cells responded but did not stimulate whereas B lymphocytes and monocytes stimulated but did not respond. NULL lymphocytes both stimulated and responded. In antibody-dependent cell-mediated cytotoxicity (ADCC), null lymphocytes and monocytes functioned as effectors, whereas T lymphocytes and granulocytes did not. B lymphocyte preparations were intermediate in activity. Coulter sizing is a rapid, simple means of assessing the purity of suspensions enriched for T, B, and null lymphocytes.