Previous work in our laboratory has shown that cytokine production by primary murine macrophages, and macrophage cell lines, is inhibited following treatment with the kappa-opioid agonist U50,488H. Furthermore, we have found that the participation of both accessory cells and T cells in an antibody response is suppressed by this compound. We have utilized the superantigen staphylococcal enterotoxin B (SEB) to further examine the effects of U50,488H on accessory and T cell function. The results showed that the proliferative response of lymph node T cells to SEB presented by activated macrophages was significantly inhibited by the kappa-opioid agonist at concentrations as low as 100 nM. However, suppression of the T cell response to SEB presented by resting macrophages required 100 times the concentration of U50,488H. On the other hand, the production of IL-2 in response to lymph node T cell stimulation with SEB was not altered by the opioid treatment. Additional experiments utilizing the opiate antagonist naloxone and the kappa-selective antagonist nor-binaltorphimine (norBNI) were performed in order to further characterize the opioid receptor involved in the suppressive activity of U50,488H. Results showed that both naloxone and norBNI were able to block the inhibitory activity of U50,488H. Further analysis showed that the proliferative response of thymic T cells was more sensitive to the effects of U50,488H, and the response with both activated and resting macrophages was suppressed. In addition, the production of IL-2 by the thymic T cells was also inhibited by the opioid treatment. The mechanism of suppression of superantigen-induced T cell responses is discussed.