The Gib 13 monoclonal antibody was raised against eggs of Onchocerca gibsoni and subsequently found to react with a phosphorylcholine epitope designated as the T15 idiotype. Since an immunoradiometric assay based on the Gib 13 monoclonal antibody holds promise for serodiagnosis of filariasis, the goals of the current study were to evaluate phosphorylcholine epitope production and release by various parasite stages and to assess changes in serum epitope levels during different phases of Brugia malayi infection in jirds. Extracts of B. malayi adult male worms, female worms, and microfilariae contained Gib 13 monoclonal antibody-reactive antigens of Mr 25-30,000, 57-90,000, and approximately equal to 200,000. Adult female worms secreted ten-fold more epitope than microfilariae on a weight basis. Phosphorylcholine-containing antigens were localized in female and male worms, respectively, in egg-bearing regions and the intestines. Assessment of the relationship between serum levels of Gib 13 antibody-binding epitope and parasitologic status of B. malayi-infected jirds showed that the immunoradiometric assay distinguishes patent infected from uninfected control animals, detects a significant rise in epitope level during the prepatent phase of infection, and is unaffected by diethylcarbamazine-induced reduction in the intensity of microfilaremia. There was a direct positive correlation between serum epitope level and female adult worm load. Quantification of serum phosphorylcholine epitope of the T15 idiotype may be useful as an indirect measure of parasite burden in humans with lymphatic filariasis that is independent of microfilaremia.