Three stages of the filarial parasite Brugia malayi (infective third stage larvae, adult worms, and microfilariae) were analyzed for differences in their protein composition by two-dimensional gel electrophoresis. Comparison of protein profiles of the different stages showed both identical polypeptides (reflecting common proteins) and polypeptides specific for each stage. Three polypeptides present only in infective stage larvae were seen at 72 kDa at an isoelectric point (pI) of 4.98 (p72), 30 kDa at pI 5.5 (p30), and 22 kDa at pI 4.75 (p22). p72 could be labeled chemically with 125I by either chloramine T or IODO-GEN and biosynthetically with [3H]leucine and [35S]methionine during in vitro culture of live larvae; thus, p72 is most likely a surface protein of parasite origin. The antigenic composition of these polypeptides was elucidated by immunoblot analysis. Both p72 and p22 were recognized by hyperimmune rabbit sera to infective larvae; sera from rabbits immunized with adult worms, however, did not recognize any of these Ag. Sera from humans infected with the related Wuchereria bancrofti filaria recognized only p72 and not the other two polypeptides. It therefore appears that p72 is a stage-specific but not genus-specific Ag that is immunogenic in the infected host. p22 also appears to be stage specific and, because it is not recognized by W. bancrofti-infected sera, it may be either a species-specific Ag or a poorly immunogenic molecule of the parasite. With mAb raised to p72, p30, and p22, these proteins were shown to share several antigenic determinants when analyzed by immunoblotting. The shared epitopes were present on numerous molecules with a wide range of apparent m.w. in each of the different parasite stages. Thus, despite the apparent larval stage specificity of these molecules themselves, they must contain certain epitopes shared by molecules from other stages as well. The identification of the p72 polypeptide as a molecule with epitopes exposed on the surface of infective larvae provides a candidate Ag for testing as a protective immunogen.