An in vitro assay was used to compare the effect of opioids on antibody production by splenocytes from C3HeB/FeJ, C57BL/6J, C57BL/6ByJ and B6C3F1/J mice immunized with sheep red blood cells (SRBC). Spleen cells were removed from mice that had been injected 2 wk prior with SRBC. These mice received no opioids in vivo. Dissociated spleen cells taken from each of the mouse strains were exposed to morphine with or without naloxone, or to U50,488H with or without norbinaltorphimine (nor-BNI), for 5 days in a Mishell-Dutton culture, with added SRBC as antigen. Immune responsiveness was assessed by the number of plaque-forming cells (PFC) per culture. The results showed a profound difference in the effects of the opioids on the spleen cells of the four mouse strains. Spleen cells of C3HeB/FeJ mice were suppressed approximately 50% in the number of PFC both by morphine (10(-5) to 10(-8) M) and by U50,488H (10(-5) to 10(-11) M). Suppression was blocked by pretreatment with naloxone or norbinaltorphimine, respectively. In contrast, spleen cells taken from C57BL/6J mice were not suppressed by either opioid, at doses ranging from 10(-5) to 10(-11) M. Spleen cells of B6C3F1/J mice were suppressed by U50,488H, but not morphine. Cells of C57BL/6ByJ mice gave inconsistent results in experiments measuring suppression by morphine, and U50,488H. Overall, these studies confirm our previous work showing that opioids directly affect the function of cells of the immune system via classical opioid receptors. In addition, the results show that mouse strain is a major variable in evaluating the immunomodulatory effects of opioids.