The presence of biological response modifiers (BRM)-like effect was confirmed in peritoneal exudate (PE) of Toxoplasma gondii-infected ICR mice which inhibited Concanavalin A (Con A)-induced peritoneal lymphocyte (PL) proliferation. During 5 days of PL incubation with 10 micrograms/ml Con A with or without PE, 3H-thymidine uptake was measured for the last 24 hrs. Compared to uninduced control, PL proliferated by 7.3-fold with Con A induction. When PE of infected mice was added, PL proliferation was inhibited by 74.0 +/- 11.9% whereas inhibition by PE of normal mice was 16.4 +/- 8.3%. Inhibitory effect of PE increased exponentially from 3 days up to 4-5 days of survival after the infection. Inhibitory activity of PE was decreased concentration-dependently. Also the inhibition was diminished when the PE was treated with heat of 95 degrees C for 10 min or precipitated with 10% trichloracetic acid (TCA). In SDS-PAGE of PE, many minor bands appeared newly. Heat-labile protein molecule in PE exerted inhibitory activity to Con A-induced lymphocyte proliferation.