Mixed leukocyte reaction suppressor factor is produced by spleen cells sensitized in vivo and restimulated in vitro across non-H-2 antigenic barriers. Cells capable of producing this factor appear in the spleens of minor locusimmunized animals later than in animals sensitized to major histocompatibility complex-encoded antigens. However, both H-2 and non-H-2-induced factors suppress proliferative responses to any alloantigen. Splenocytes from animals immunized with H-2-identical, minor locus-disparate cells produce suppressor factor in vitro only wlien restimulated with cells sharing both H-2 and non-H-2 antigens with the in vivo stimulators. © American Society for Clinical Pathology.