Survival of B10.BR mice transplanted with C57.BL6 BM cells was enhanced by irradiated C57.BL6 splenocytes. Data are pooled from 3 experiments in which groups of 8 to 10 B10.BR mice were irradiated to 10 Gy on day -2 and transplanted with 0.5 × 10⁶ C57.BL6 allogeneic BM cells or 0.5 × 10⁶syngeneic B10.BR BM cells on day 0. One group of 8 B10.BR mice received 3 injections of 25 × 10⁶ nonirradiated C57.BL6 allogeneic splenocytes on day −1, day 0, and day +1 (▴), while a second group of 28 B10.BR mice received 3 injections of 25 × 10⁶irradiated (7.5 Gy) allogeneic C57.BL6 spleen cells on day −1, day 0, and day +1 (•). A control group of 60 B10.BR mice received allogeneic BM cells alone (▪). Another control group of 20 B10.BR mice received irradiation without a transplant (□) and 26 B10.BR mice were transplanted with syngeneic H2^k BM cells (⧫). Significant differences in survival using the Cox statistic were found between the group of mice receiving multiple infusions of irradiated allogeneic splenocytes and the group that received nonirradiated splenocytes (P = 3 × 10⁻⁶); the mice that received irradiated allogeneic splenocytes and mice that received allogeneic BM alone (P = 1 × 10⁻²); and the mice that received multiple infusions of nonirradiated allogeneic splenocytes and mice that received allogeneic BM alone (P= 3 × 10⁻⁷).