Having previously established, that prostaglandins play a role in the regulation of the immune response to polyvinyl pyrollidone, a T-independent antigen, further investigations of the role of prostaglandins and cyclic nucleotides in the control of the immune response to polyvinyl pyrollidone were initiated. Strongly immunogenic (PVP 360,000) and weakly immunogenic (PVP 10,000) molecular sizes of polyvinyl pyrollidone were examined for their effects on splenic PGF2 alpha, PGE and cyclic nucleotide levels. The results show, that PVP 360,000 induces marked changes in PGF2 levels. There is an early marked depression at 2 hours after injection followed by an increase which peaks at 2 hour. At subsequent time intervals (9-10, 13-14 and 16-18 hour) high values were observed, especially in the latter case. cAMP levels undergo significant fluctuations, exhibiting very big rise at 12 and 13 hour post-immunization, cGMP levels are elevated at 2 hour declining thereafter. PGE level in C57Bl mice exhibits very substantial increase at 4-6 hour after immunization, in athymic mice, however, the increase was not significant and was preceded by a profound drop in PGE concentration. PGE level in the splenocytes from athymic mice shows a constant increase till 4 hour after PVP addition, followed by a little decrease at 6-7 hour. cAMP concentration in athymic mice exhibits a drop at 3-4 hour after immunization, followed by an increase at 5-6 hour post-immunization. Indomethacin, an inhibitor of prostaglandin synthetase, blocks the changes in PGF2 and cGMP level but has little effect on cAMP. In contrast, the weakly immunogenic form PVP 10,000 induces a large bimodal increase in cAMP levels peaking at 2 hour and again increasing between 6-8 hour; cGMP levels also rise, but more slowly. The increase in cAMP is blocked by indomethacin even though no comparable increases in PGF2 levels are observed. The changes induced by PVP 10,000 appear to be dependent on T cells since comparable changes are not observed in athymic mice. Although PVP 10,000 is non-immunogenic in normal mice or whole spleen cultures, it is immunogenic in athymic mice and purified B cell cultures. This difference has been traced to an apparent difference in the activation of T cells vs. B cells by PVP 10,000. Lastly, although inhibition of PG synthesis results in an enhancement of the immune response to PVP 360,000, no such enhancement is observed with PVP 10,000. The relevance of prostaglandin and cyclic nucleotide changes to the development of the immune response is discussed.