In vivo immunization of normal volunteers with tetanus toxoid induces the formation of a circulating B cell subset that has the capacity to secrete specific antibody in vitro without the need for T cell help or mitogen stimulation. From earlier studies it was not clear whether the spontaneous antibody-secreting lymphoblastoid (LB) B cell was at a terminal stage of differentiation or if it had the capacity to give rise to additional B cell subsets, such as memory cells or more fully mature antibody-producing cells. In this study we have shown that at least two distinct waves of spontaneous antibody secretion can occur in vitro when cultures are initiated with the lymphocytes from individuals immunized 6 days earlier. The first production of antibody was completed by 3 days of in vitro culture and the second production of antibody did not initiate until day 7 or 8 of culture and was completed by day 12. The B cells responsible for the second stage of antibody production appeared derived from a portion of the antibody-secreting cells present on day 3 in that 1) treatment of the cultures on day 0 with BuDr and light equally inhibited the first and second rounds of antibody synthesis; 2) when isolated from the blood, both B cell subsets were in the large cell fraction after 1 X G sedimentation; and 3) under conditions of limiting numbers of cells, the cells responsible for the second wave of antibody production were almost exclusively found in cultures positive for a B cell that had produced antibody on days 1 to 3. Although only a portion (10 to 30%) of the LB B cells present on day 0 had the capacity to again produce antibody on days 8 to 12, the two cells were capable of producing similar quantities of antibody on a per cell basis. These results indicate that the mature circulating LB cell induced in vivo by immunization is not terminally differentiated, but under appropriate conditions has the capacity to give rise to additional antibody-secreting cells.