We have determined that suppression of the in vitro T-dependent humoral immune response by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is dependent on the type and concentration of serum used in the culture media. Only five out of 23 commercial lots of serum (screened at both 5 and 10%) could support a suppression in the presence of 30 nM TCDD, with the remaining lots demonstrating an apparent 'protective-like' effect against the TCDD exposure. When log dose response curves were established with TCDD (0.3, 3.0, and 30 nM) in media containing each of the serum lots supporting a suppression (at both 5 and 10%), we determined that only three lots could support a full dose-responsive suppression. Subsequently, in a comparison study between the effects of TCDD and the polyclonal B-cell activator Staphylococcus aureus Cowan Strain I (SAC) on the in vitro T-dependent humoral immune response, we have found that SAC suppresses the antibody response to SRBC and demonstrates the same serum dependency for this effect as was previously noted for TCDD. Under serum-free culturing conditions, TCDD (30 nM) caused a 15-fold increase in the AFC response to SRBCs over controls, suggesting that direct addition of TCDD to whole splenocyte cultures in the absence of serum-derived growth factors results in an increase in B-cell activation. Likewise, under serum-free conditions, SAC dose-dependently increased the AFC response over media controls, and at doses which achieved the same degree of suppression of the humoral response aa TCDD. Taken together, these studies suggest that TCDD has actions that are similar to a T cell independent polyclonal B cell activator such as SAC, and selectively acts on the B cell to suppress the T-dependent humoral immune response by a mechanism which is unique to this series of compounds. This effect however, is only detectable under appropriate serum-supported (or serum-deficient) culture conditions as described.