The present studies tested the effect of withdrawal from morphine by two different paradigms, abrupt withdrawal (AW) or precipitated withdrawal (PW), on the capacity of murine spleen cells to mount an in vitro antibody response. Mice were made dependent by chronic treatment using s.c. implanted morphine slow-release pellets. Splenocytes were harvested at various time points after withdrawal and the number of antibody-forming cells determined using a plaque-forming cell (PFC) assay. The results indicate that induction of abstinence from morphine in dependent mice by either paradigm caused marked immunosuppression between 24 and 48 h post-withdrawal. However, the kinetics of onset and recovery from immunosuppression were different in AW and PW.