Summary: Hemolytic antibody-forming cells from spleen, lymph nodes, blood and thoracic duct lymph of the rat have been analyzed and in some cases purified using the technique of equilibrium density gradient centrifugation. Data on the size range distribution and kinetics of appearance have also been obtained. The results are in accord with the pleomorphism of 19 S hemolytic antibody-forming cells established by morphologic criteria. The density distribution profiles are extremely complex and do not simply represent a continuum of components. The results provide evidence for distinct stages rather than a continuing process of development of the antibody-forming cell. Differences in the density profiles between fixed and circulating antibody-forming cells suggest a restriction in the classes of cells that can enter the circulation. One circulating antibody-forming component was enriched 100-fold by density separation whereas no more than two-fold enrichment was obtained with cells from spleen or the lymph nodes. The technique offers promise as an analytic approach to cell population dynamics.