The surface antigens of Toxocara canis infective larvae have been identified by radio-iodination and compared with the excretory-secretory (ES) products released by the larvae in vitro. Common antigens, of molecular weight 32 000 and 120 000 are found on the larval surface, in the ES material and in culture supernatant following surface iodination of living T. canis larvae. The 120 000 antigens consist of three closely migrating bands in each of these preparations. However, one prominent ES component, of molecular weight 400 000, is not found on the larval surface. Additional molecules of 55 000 and 70 000 are present in the ES material, but while these may be discerned in surface preparations there appears to be more heterogeneity of surface molecules in this size range. Both sets of molecules are antigens to infected patients and experimental animals. A comparison of characterized human sera show that a radio-immunoprecipitation assay correlates with the established ELISA test (r = 0.89), and that all labelled molecules are antigenic to the infected host.