Within 2 min following the intravenous injection of sheep erythrocytes (sRBC) there occurs 20 to 80-fold increase in prostaglandin (PG) F2alpha in the spleen. This burst of synthesis is followed by a slow decline to control levels over the next 1-4 hr. No increase in splenic PGF2alpha levels is observed between 24 and 72 hr after injection. Injection of colloidal carbon results in a small increase, approximately 20% of the increase in PGF2alpha observed with sRBC. The early increase in splenic PGF2alpha levels stimulated by sRBC is also dependent upon thymus-derived (T) cells, since the increase is small or nonexistent in athymic mice and NZB mice. Also, the elevation of splenic PGF2alpha levels is blocked by the administration of indomethacin or Ro 20-5720, both of which block the synthesis of prostglandin. A small increase (2-fold) in PGF2alpha levels occurs in the thymus. A soluble antigen, bovine gamma globulin, stimulated a bimodal increase in splenic PGF2alpha levels, the early peak occurring at 2 hr and the later increase occurring at 48 hr. Using inhibitors of prostaglandin synthesis, it is possible to enhance the appearance of cells forming 19S antibody against sRBC, both in vivo and in vitro. Furthermore, inhibition of prostaglandin synthesis enhances DNA synthesis induced in a two-way mixed-lymphocyte reaction only in whole spleen cell cultures and not in cultures of spleen cells purified by passage over glass wool. Based on this evidence, it is proposed that the prostaglandins represent a major soluble mediator in the control of T cell-T cell interactions and also play an important part in T-B (bone-marrow derived) cell interactions.