Both in vivo and in vitro exposure to morphine have been reported to produce a number of immunomodulatory effects in both laboratory animals and humans. The current study was performed to assess the direct in vitro effect of exposure to morphine or morphine metabolites on immune response parameters. Murine B6C3F1 splenic lymphocytes or peritoneal macrophages were cultured in vitro at concentrations of 0.0001-100 mumol/l morphine sulfate, morphine-3-glucuronide, morphine-6-glucuronide, or normorphine. B cell proliferation was significantly suppressed following exposure to all drugs. Production of interleukin (IL)-2, IL-4, and IL-6 was affected only moderately by all drugs except morphine-6-glucuronide, which produced a marked suppression at 100 mumol/l. Both basal and augmented natural killer (NK) cell function were unaffected by any drug except morphine-6-glucuronide, which enhanced NK cell activity at concentrations between 0.0001 and 1.0 mumol/l. In contrast, both morphine-3-glucuronide and morphine-6-glucuronide significantly inhibited cytotoxic T lymphocyte induction at concentrations between 0.0001 and 100 mumol/l, whereas morphine and normorphine were inactive in this assay. In summary, in the absence of direct cellular cytotoxicity, a differential immunomodulation was observed following in vitro exposure to morphine and its metabolites.