The BSA density-gradient technique for separating mouse spleen cells into partially purified populations has been used to compare the responsiveness of such populations to SRBC using in vivo and in vitro techniques. Two major populations were distinguished, one of which responded very well in vivo with an exponential dose response and poorly in vitro (fraction 3), and another which responded in vivo and in vitro with a linear dose response (fraction 2). A light density, radiation-resistant component was identified which markedly stimulated the response of fraction 3 in vitro, and a density gradient profile was obtained for this cell which did not correspond with a macrophage profile. A high density, radiation-sensitive cell was identified which stimulated the response of PFC precursors in lighter regions of the gradient. The activity of this cell could be replaced using thymus cells. A density profile for the PFC precursor cell was obtained by assaying small numbers of spleen cell fractions in the presence of an excess of the two auxiliary cell types
