Antigenic proteins of microfilariae and infective larvae of Wuchereria bancrofti have been identified by intrinsic and extrinsic radiolabelling, and specific immunoprecipitation with sera from filarial patients. From 125I surface-labelling experiments, the most prominent antigen on both stages is of relative molecular mass (Mr) 17 000, while a molecule of similar size is both synthesized and released in vitro following labelling with [35S]methionine. A second similarity between the two stages is the production and secretion of a Mr 21 000 component, which is, however, not detected on the worm surfaces. A series of additional proteins from larval W. bancrofti are described from each parasite compartment (secreted, surface and somatic) and the antigenicity and specificity of these components explored with serum from patients with filariasis due to W. bancrofti or Brugia species, and with onchocerciasis. Among additional molecules released in vitro we have found a Mr 51 000 antigen from both stages, and also several proteins which are not recognised by antibody from human filarial patients.