Previous studies have indicated that mice which differ in their acute susceptibility to responses mediated by the Ah receptor have a pattern of suppression of the antibody response which is consistent with a role by the putative dioxin receptor. The objective of the present investigation was to compare the TCDD-induced suppression of the antibody response following acute and subchronic exposures in B6C3F1 mice, an Ah-high-responder strain, and DBA/2 mice, an Ah-low-responder strain. Results of our initial studies demonstrate that suppression of humoral immunity can be enhanced in DBA/2 mice approximately 10-fold following subchronic versus acute exposures to the same cumulative doses of TCDD. This change in suppression of the antibody response in DBA/2 mice was not accompanied by significant changes in liver weight (hepatomegaly), as was observed in the B6C3F1 strain when exposed under comparable conditions. In contrast, effects on thymus weight (involution) were enhanced in the DBA/2 mice following subchronic exposure and demonstrated a higher degree of atrophy than was seen in the B6C3F1 strain (68 versus 56% decrease in thymic weight at the 42 micrograms/kg cumulative dose). These findings suggest that multiple mechanisms may be operating to suppress humoral immunity in vivo and that the conditions of exposure can alter the toxic effects of TCDD in the DBA/2, Ah-low responsive, mouse strain.