The present study was undertaken to investigate the effect of biphenyl dimethyl idcarboxylate (PMC) on the humoral immunosuppression by ketoconazole (KCZ) in ICR mice. PMC at a dose of 6 mg/kg was administered orally to mice daily for 14 consecutive days. KCZ was suspended in RPMI 1640 medium and orally administered at 160 mg/kg/day 2 hrs after the administration of PMC. Mice were immunized an challenged with challenged red blood cells (SRBC). The results of the present study are summarized as follows; a gain of body weight and relative weights of spleen and liver were significantly increased by combination of PMC and KCZ, as compared with those in mice treated with KCZ alone. Splenic plaque forming cells (PFC) and hemagglutination (HA) titers to SRBC were greatly enhanced by the combination of PMC and KCZ, compared with treatment of KCZ alone. The elevation of serum glutamicpyruvic transminase (S-GPT) and total protein levels caused by KCZ were reduced to normal level by the combination of PMC and KCZ. In addition, lower serum albumin and A/G ratio were also increased to normal level. These findings indicate that PMC has a protective effect against KCZ-induced humoral immunosuppression.