Somatic antigens of Loa loa adult worms with molecular weights of 15–180 kDa were identified by Western blot analysis using sera from 3 categories of parasitologically and clinically defined subjects from a loiasis endemic zone. Sera of occult, amicrofilaremic (OL), and ‘resistant’ individuals with no clinical signs of infection (R) reacted with an antigen of 160 kDa; sera of highly microfilaremic individuals (ML) did not. ML sera strongly reacted with an antigen of 18 kDa which was recognized only weakly or not at all by OL and R sera. At higher dilutions, OL sera only reacted with antigens at 23 and 160 kDa and ML sera reacted with antigens at 18 and 23 kDa, whereas R sera reacted with antigens at 23, 42, 54, 70, 100, and 160 kDa. These data suggested that R sera contained a higher concentration of antibodies which reacted with denatured, nitrocellulose-bound antigens. The IgG4 isotype predominated for all groups of sera, while IgG3 antibody responses were observed only with R sera. IgG1 antibodies were seen in all groups but reacted with fewer antigens than IgG4 antibodies, and no IgG2 antibody responses were detected. Sera against Brugia malayi, Wuchereria bancrofti, Onchocerca volvulus, and Dirofilaria immitis cross-reacted with somatic antigens > 70 kDa, whereas none reacted with Loa loa antigens <23 kDa.