The BALB/c (H-2d Mls2)-DBA/2 (H-2d Mls1) mouse model was used to examine whether Mls loci stimulated reactions in mixed lymphocyte culture (MLC) could be used to monitor cellular interactions governed by other non-H-2-encoded antigens in graft rejection. Several new findings have emerged: (1) Cells primed in MLC and then irradiated (x) were found to strongly stimulate fresh syngeneic splenocytes, as well as potentiate and accelerate the response of fresh allogeneic splenocytes in MLC. (2) Potentiation was also produced by primed cells added as third components to MLCs made up of fresh cells. This was magnified if the stimulating cell population was the same as that to which the primed cells were directed. (3) These primed cells used as stimulating cells in syngeneic or allogeneic cultures with fresh responding cells could be used to generate entirely in vitro a second population of blasts that exhibited suppressor cell activity. A two-step process was required to evoke these suppressor cells: (1) BALB/c (B) cells were stimulated with DBA/2X (Dx) to provide a primed anti-DBA (B'D) blast population; and (2) when B'Dx cells were then used as stimulating cells in MLC with fresh syngeneic BALB/c or allogeneic DBA/2 as responders, a suppressor cell population B'B‘D or D'B’D was generated in the putative absence of cytotoxicity. Preliminary results indicate that survival of DBA/2 skin allografts can be prolonged by immunizing BALB/c recipients with B'Dx in complete Freund's adjuvant. The implications of these findings are discussed.