In this study we investigated the capacity of morphine to modulate expression of cytokines in peritoneal macrophages. Mice were implanted subcutaneously with a 75-mg morphine slow-release pellet, and 48 h later resident peritoneal macrophages were harvested. Control groups received placebo pellets, naltrexone pellets, or morphine plus naltrexone pellets. Adherent cells were stimulated with lipopolysaccharide (LPS: 10 microg/mL) plus interferon-gamma (IFN-gamma: 100 units/mL) to induce cytokine production. After 24 h RNA was extracted for analysis of cytokine mRNA levels by reverse transcriptase-polymerase chain reaction, or supernatants were collected after 48 h for determination of cytokine production by enzyme-linked immunosorbent assay (ELISA). Morphine enhanced mRNA expression of interleukin (IL)-12 p40 and tumor necrosis factor alpha (TNF-alpha) compared with controls, whereas IL-10 levels were unchanged by drug treatment. ELISA data showed that both IL-12 p40 and p70 were increased by morphine. The enhancement of IL-12 at both the mRNA and protein levels was antagonized by naltrexone, indicating that the modulation of this cytokine by morphine is via a classic opioid receptor. These results are particularly interesting in light of our previous observation that 48 h after morphine pellet implantation, the peritoneal cavity is colonized with gram-negative and other enteric bacteria. The enhancement of IL-12 by morphine might be related to morphine-induced sepsis.