The immunosuppressive effects of in vivo (subcutaneous) exposure to 40 or 80 mg/kg 3-methylcholanthrene (MC) were examined in aryl hydrocarbon hydroxylase (AHH) responsive C57BL/6 (B6) and AHH non-responsive DBA/2 (D2) inbred strains of mice. Twenty-four hours after treatment with carcinogen or vehicle alone, animals were primed with crude L1 muscle larvae antigen from T. spiralis. Immune status was assessed in vitro after six days as antigen-specific lymphoproliferation. The proliferation of splenocytes from MC-treated D2 and B6 mice was significantly impaired compared to controls. To examine the cellular basis of the immunosuppression, primed splenocytes from control and MC-treated mice were separated into adherent and non-adherent fractions on Sephadex G-10 columns. When antigen-pulsed adherent cells from MC-treated B6 and D2 mice were recombined with control non-adherent cells from syngeneic and B6D2F1 mice, T-cell proliferation was significantly reduced. This suppression was not observed with the addition of increased numbers of adherent cells. Non-adherent cells from MC-treated mice showed a decreased capacity to respond to the presence of control antigen-pulsed adherent cells from appropriate mice. These results suggest that MC treatment has a similar suppressive effect on the immune responses of both B6 and D2 mice that involves the quality of accessory cell-T-cell interactions.