The role of cellular proliferation and recruitment was evaluated in the spleens of mice undergoing a primary immune response at 2-hr intervals during the first 96 hr after antigenic stimulation. The response was further analyzed autoradiographically and by the use of vinblastine, a mitotic inhibitor. It was found that: a) antigen-induced cellular proliferation began about 12 hr after antigen injection; b) plaqueforming cells (PFC) began to appear at a level significantly above background following a lag phase of about 24 hr; c) most, if not all, PFC during the log phase of the response are themselves proliferating or are the progeny of proliferating precursor cells; d) the number of PFC increased in a staircase manner, suggesting a considerable degree of synchronous growth; and e) there was a series of recruitment events occurring nonrandomly and in phase with division of PFC. These findings establish the essential and dominant roles of cellular proliferation during the log phase of the primary response. An important and unexpected observation is that there are many recruitment events occurring in a nonrandom manner and that the number of cells recruited increases exponentially. These results indicate that care should be taken in evaluating kinetic indices derived from analysis of data based on the assumption that only one recruitment event is involved and that the growth in the number of antibody-forming cells is random.