The alloantigenic reactivity of hamster spleen cells from unsensitized animals was analyzed in vitro. When compared with normal MHA cells, MHA anti-CB spleen cells responded better or to the same degree in mixed lymphocyte reaction (MLR) during the first 3 days after alloantigenic exposure. However, by day 5 postsensitization there was a signigicant reduction in spleen cell MLR activity, which returned to normal or even an exaggerated response 7 days following immunization. We examined the possibility that the reduction in MLR responsiveness 5 days following immunization might be attributable to the generation splenic suppressor cells. Irradiated spleen cells from unsensitized and from MHA anti-CB donors were cocultured with normal responder MHA lymph node cells stimulated by (MHA x CB)F1 hybrid lymph node cells in MLR. No regulatory effect of either cell population was seen, indicating that even hamster spleen cells from alloimmune donors fail to suppress the mitotic response of normal lymph node cells to allogeneic targets.