BALB/c mice immunized with L3 of Brugia pahangi, irradiated at 45 kRad from a 137Caesium source, are strongly immune to challenge infection (75-100% reduction in the recovery of challenge infection larvae on day 6 post-challenge). The target of immunity appears to be the post-infective L3, as challenge infection larvae are killed within 5-6 days of infection. By immunoblot analysis, serum from immune animals recognizes a limited set of somatic antigens, the majority of which are shared between different life cycle stages. Serum from immune mice also strongly recognizes larval surface antigens by immunofluorescence, some of which may be stage specific. The larval surface determinants do not appear to be protein or glycoprotein by standard immunochemical analysis. A proportion of the antibody response of the BALB/c mouse is directed towards phosphorylcholine epitopes on filarial antigens, but the limited antigen recognition cannot be explained on the basis of the mouse strain used, as CBA/Ca mice recognize a similar limited set of antigens.