IL-10 has a variety of effects including: inhibition of monocyte MHC class II-dependent Ag presentation, Th1 cytokine production, and inhibition of T cell proliferation. Recently we have shown that IL-10 inhibits Ag presentation to human tumor-specific and allospecific CTL. In the present study we showed that transfection of the mouse lymphoma RMA (H-2b) with the IL-10 gene induced conversion to a RMA-S-like phenotype. The changes included an inhibition of lysis by minor histocompatibility or tumor Ag-specific CTLs and, conversely, a dramatic increase in susceptibility to lysis by NK cells. The RMA-10 transfectants showed levels of H-2 expression as low or even lower than those found on RMA-S. The levels of tested adhesion molecules were unaltered. Treatment of RMA with rIL-10 gave a less pronounced change in phenotype. In addition, relative to untreated target cells, IL-10 pretreated cells or IL-10 transfectants were unaltered in their capacity to affect cytotoxicity by cold target inhibition, arguing against the possibility that the observed effect could be a direct effect of IL-10 on the CTL. The expression of H-2 was partially restored by coculturing RMA-10 transfectants with class I-binding peptides. Taken together, these results indicate that IL-10 exerts a post-transcriptional effect on H-2 expression, compatible with an induced decrease in the access of peptides to the MHC class I complex. IL-10 is the first cytokine reported to have this effect and also the first factor shown to induce NK sensitivity and reduced sensitivity to CTL, an effect that may be of physiologic relevance.