Direct addition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 5-20 nM) to cultures of spleen cells from (C57BL/6 x C3H)F1 (B6C3F1) mice produced a suppression of the number of antibody-producing cells which developed in response to lipopolysaccharide, dinitrophenyl-Ficoll and sheep erythrocytes. The suppression of all three parameters was dose-related and parallel. This parallelism and the observation that the magnitude of the suppression was comparable in all three models suggested that the B-lymphocyte was the primary target. The defect was attributed to an effect on early activation or impaired differentiation because direct addition of TCDD had no effect on mitogen-induced proliferation. Temporal studies showed that TCDD produced the greatest suppression of the polyclonal antibody response to lipopolysaccharide when added at the beginning of the culture and that there was no suppression when TCDD was added as soon as 3 h after 200 micrograms/ml lipopolysaccharide. The observation that TCDD could directly suppress the antibody response by spleen cells from DBA/2 mice, at concentrations comparable to those required to suppress the B6C3F1 mice, suggested that the effect on the B-lymphocyte was atypical of the profile of activity (i.e., dependence on the Ah locus) previously reported to characterize the effects of dioxin in other systems. Similar results were demonstrated with congenic mice, as Ahd/d homozygotes were suppressed comparably to Ahb/d heterozygotes. The direct suppression by 2,7-dichlorodibenzo-p-dioxin, a congener previously demonstrated to be devoid of affinity for the Ah locus, further suggests a dissociation from the traditional profile of activity.