We investigated the effects of retinoic acid (RA) on natural killer (NK) cell activity and on the susceptibility of various tumor target cells to NK cell lysis. These studies were undertaken to assess the overall effects of RA treatment on the natural killing of tumor cells in humans. We also evaluated how RA affects the generation of NK-like activity in mixed leucocyte culture, to determine whether or not this compound can influence the regulation of natural cytotoxicity during the induction phase of NK cell development. Our results indicate that pharmacological levels of RA have little, if any, influence on the development, cytotoxic potential, or activity of NK cells in humans. In contrast, RA can significantly reduce the sensitivity of some tumor target cells to natural killing. This effect appears to be directly related to the differentiation-promoting properties of RA, since reduced NK susceptibility was observed with target cells that were "differentiated" by RA treatment, but not with cells that were either unaffected by RA or were only growth inhibited without concomitant differentiation. These findings indicate that the immunomodulating properties of RA do not extend to human NK cell activity. However, a possible decreased susceptibility of tumor cells to natural killing should be a consideration when planning therapeutic applications of RA in certain cancers.