The immunological basis of resistance to Brugia pahangi infection in rats was studied. Infections were investigated in athymic rnu/rnu rats and in rats treated with the immuno-suppressive agents cyclosporin A (CsA) or cyclophosphamide (Cy). The recovery of adult worms in normal rats was 1-2% in comparison to 12.2% recovery in athymic rats. CsA and Cy treated rats did not have increased adult worm burdens. Microfilarial (Mf) levels (expressed as Mf per ml per adult worm) were highly elevated in both athymic and Cy treated rats but not in CsA treated rats. IgG and IgM levels to B. pahangi antigens were severely depressed in both athymic and Cy treated rats. IgG levels but not IgM levels were abrogated in CsA treated rats. These results implied that control of larval establishment or adult killing, and regulation of Mf levels are separate T-cell dependent mechanisms and act independently of IgG antibody. Control of Mf levels is associated with a specific IgM response which is Cy sensitive but CsA resistant.