The filarial parasite of cattle, Onchocerca gibsoni, has been used to establish procedures of antigen identification with a view of applying these techniques to studies on human filarial parasites. Emphasis has been placed on methods suitable for use with small numbers of parasites. Microfilariae (mf) of O. gibsoni were extracted from nodular worms, purified and 125I-labeled using IODO-GEN in solid-phase. Radioactivity was shown to be confined to the cuticle of sectioned mf using the technique of electronmicroscope autoradiography. Radiolabeled mf were analysed by two-dimensional gel electrophoresis. Autoradiographs of 125I-labeled proteins of O. gibsoni mf were relatively complex, there being at least 32 proteins ranging in molecular weights from 20,000 to 120,000 and displaying considerable charge heterogeneity. Evidence was obtained that at least the major serum proteins of the host, albumin or immunoglobulin, were not absorbed on the surface of these uterine mf and detectable in the labeled surface protein patterns. Sera from infected cattle immunoprecipitated 5 labeled proteins from a Triton X-100 extract of 125I-labeled mf. Sera from either of two calves which had been given multiple injections of mf subcutaneously, and which had no detectable skin mf, recognised 6 additional proteins in this extract as well as 3 of the proteins recognised by sera from infected cattle.