BALB/C mice were intraperitoneally injected with methamphetamine (5 mg/kg) to observe the effect of methamphetamine on the immune system. Body weights were decreased in both acutely treated group (twice for 2 weeks with 7 days interval) and subchronically treated group (daily injection for 14 days). The relative spleen weights and the numbers of splenocytes were unexpectedly increased (p<0.05) in acutely treated group, but subchronically treated group showed the trend of decrease without significance. But there was no significant effect on antibody formation to hen egg Iysozyme which was immunized during the treatment of methamphetamine and on plaque forming cell number. The relative thymus weights of both groups were significantly decreased by the treatment of methamphetamine (acutely treated group, p<0.05; subchronically treated group, p<0.01). These results suggest that the effect of methamphetamine on the immune system may be caused by thymic dysfunction.