The nematode surface coat is defined as an extracuticular component on the outermost layer of the nematode body wall, visualized only by electron microscopy. Surface coat proteins of Meloidogyne incognita race 3 infective juveniles were characterized by electrophoresis and Western blotting of extracts from radioiodine and biotin-labeled nematodes. Extraction of labeled nematodes with cetyltrimethylammonium bromide yielded a principal protein band larger than 250 kDa and, with water soluble biotin, several faint bands ranging from 31 kDa to 179 kDa. The pattern of labeling was similar for both labeling methods. Western blots of unlabeled proteins were probed with a panel of biotin-lectin conjugates, but only Concanavalin A bound to the principal band. Nematodes labeled with radioiodine and biotin released (1)(2)I and biotin-labeled molecules into water after 20 hours incubation, indicating that surface coat proteins may be loosely attached to the nematode. Antiserum to the partially purified principal protein bound to the surface of live nematodes and to several proteins on Western blots. Differential patterns of antibody labeling were obtained on immuno-blots of extracts from M. incognita race 1, 2, and 3; Meloidogyne hapla race 2; and Meloidogyne arenaria cytological race B.