A rat model of combined pancreas-spleen transplantation (PST) was used in order to characterize the immunologic consequences of PST when compared to pancreas transplantation (PT) alone. Weakly MHC disparate Fischer (F344) PST grafts survived significantly longer in LEW recipients than did F344 PT grafts (17.6 +/- 3.4 vs 12.1 +/- 1.0 days, respectively, P less than 0.001). However, graft versus host disease (GVHD) occurred regularly in the PST recipients. Similarly, in haploidentical LBN to LEW donor-recipient pairs, PST graft survival was also modestly but significantly increased over that of the PT controls (10.6 +/- 1.0 vs 8.5 +/- 0.8 days, respectively, P less than 0.001). Conversely, in the ACI to LEW combination where MCH differences are very strong, PST graft survival was not longer than PT controls (7.5 +/- 0.8 vs 7.0 +/- 0.6 days, respectively, P greater than 0.2). GVHD was not observed in either of the latter two experiments. Short-term immunosuppression with cyclosporine further improved the outcome in LEW recipients of F344 grafts by inducing long-term graft survivals in approximately one-fourth of the PST recipients. Host splenectomy did not improve graft survival in PST recipients but did increase the risk of GVHD in LEW recipients of F344 PST grafts. Graft irradiation prior to transplantation with 500 rad not only abrogated the GVHD potential of the F344 PST graft but also eliminated the graft survival prolonging effect of the donor spleen. Donor spleen cells injected at the time of PT in F344 to LEW transplants resulted in graft prolongation not different from spleen intact PST recipients.(ABSTRACT TRUNCATED AT 250 WORDS)