This article summarizes some of the immunological activities of enkephalins and endorphins. Human and animal lymphocytes possess receptors for endorphins and enkephalins. These opioid peptides enhance T cell mitogen response. They decrease in vitro antibody formation. Endorphins and enkephalins will enhance human natural killer activity. This effect may be mediated by several mechanisms including the production of factor(s) enhancing natural killer activity by lymphocytes incubated in presence of opioid peptides. All these results suggest that opioid peptides may be activation molecules for the immune system and as such immunomodulatory agents.