Pharmacologic quantities of prostaglandin alter the immune complex nephritis of NZB/W mice. To study the mechanism of this change, NZB/W mice received 200 micrograms. of prostaglandin E1 or E2 twice daily starting at 2, 4, or 6 months of age. Mice were sacrificed at bimonthy intervals, renal function and serologic parameters were evaluated, and renal tissue was examined by light, fluorescence, and electron microscopy. Therapy decreased the incidence of proteinuria, lessened renal pathology, and prolonged survival. Maximal beneficial effects occurred when treatment began at 2 months of age. The most striking change was a decrease in the rate of immune complexes depositing in the mesangium and their absence from peripheral loops. Accompanying this change was a reduction in glomerular hypercellularity and a decrease in renal perivascular and interstitial mononuclear infiltrates. By contrast, treatment did not alter serum levels of immunoglobulins, antinuclear antibodies, and antisingle or double-stranded DNA. These results indicate that prostaglandin E is capable of prolonging survival in NZB/W mice by decreasing the rate of immune complexes depositing in glomeruli.