We report the use of unit gravity sedimentation with a CelSep apparatus to generate two volumetrically similar but functionally and phenotypically distinct subsets of human peripheral blood B cells. One subset, comprised of small B lymphocytes, underwent a significant size change in response to anti-mu, proliferated synergistically to low concentrations of anti-mu plus B cell growth factor (BCGF) or phorbol myristate acetate plus BCGF, and could be induced to produce immunoglobulin in response to pokeweed-mitogen-derived T-lymphocyte-replacing factors. These cells were primarily sIg+, B1+, B2+, and were virtually free of monocytes (less than 0.01%). Unlike these resting B lymphocytes, the large cells proliferated directly to BCGF, without displaying synergy with anti-mu. These cells displayed very little B2 (less than 7%), did not increase in volume in the presence of anti-mu, and made more immunoglobulin in response to TRF than the small resting B lymphocytes. However, neither population synthesized immunoglobulin spontaneously. This technique, which is highly reproducible, not equipment intensive, and produces high cell recovery (greater than 90%), allows for a precise analysis of the steps involved in the maturation of a resting B lymphocyte to an immunoglobulin-secreting cell.