In order to investigate the of effects of nalbuphine on immune system in mice, we examined the various immunological parameters. After single oral administration of nalbuphine (130, 260, 390 mg/kg, i.p.) to female ICR mite, the weights of bodies and organs (thymus, spleen, liver, kidney), and hematological parameters were examined on day 2, 4, 6, and 8. The increased rate of body weight, relative weight of organ, and hematological parameters in nalbuphine -treated groups, were not significantly changed when compared with control group. However, number of WBC was decreased by the treatment of nalbuphine. To assess the effects of nalbuphine on humoral immune responses, splenic IgM plaque forming cell (PFC) and serum IgM were assayed. When nalbuphine wat administered after immunization with SRBC, but not before immunization, splenic IgM PFC and ,serum IgM level against SRBC were significantly lowered in a dole -dependent manner. These results indicate that the suppressive effects of nalbuphine on primary humoral immune response may be dependent on the timing of its administration relative to the initial antigenic sensitization.