Tetanus toxoid (Tet) booster immunization induces the transient appearance in the circulation of lymphoblastoid (LB) B cells which spontaneously produce anti-tetanus toxoid IgG antibody (IgG-Tet) during a 3-day in vitro culture. In this study we have examined the effects of TPA on the ability of LB cells to secrete antibody and have found that as little as 10 ng/ml of TPA provoked a marked inhibition of the induced LB cells' IgG-Tet production. This inhibitory effect was observed only when TPA was added early in the culture and could be achieved by pretreating the B cells with TPA for as little as 1 hr. Only marginal inhibition of IgG-Tet production was observed if the addition of TPA was delayed 14-24 hr. The TPA inhibition was not mediated by contaminant T cells as the addition of increasing numbers of T cells to LB cell cultures proportionally reversed the TPA inhibitory effect. Likewise, the inhibition of antibody synthesis was not due to a monocyte-dependent mechanism since (1) substantial depletion of adherent cells did not reverse the inhibition of antibody synthesis, (2) the addition of a monocyte-enriched population to monocyte-depleted B cells did not enhance, but in fact partially reversed, the inhibition caused by TPA, and (3) the addition of monocyte populations pretreated with TPA to monocyte-depleted B-cell fractions did not inhibit subsequent IgG-Tet production by the LB cells.