The excretory/secretory (ES) antigens of the infective (L2) and lung-stage (L3/4) larvae of Ascaris have been characterized by radio-iodination, immunoprecipitation, and SDS-PAGE. These antigens were found to be heterogeneous, ranging in apparent molecular weights from 14,000 to 410,000, were stage-specific and were the targets of considerable antibody responses mounted in infected hosts. A major contaminant of the ES of L3/4 recovered from the lungs of host animals was found to be serum albumin, which appeared to have been processed in some way by the worm or its secretions. A comparison between the humoral responses of three species of experimental animals showed that rabbits recognized all ES components, while mice and rats responded to distinct subsets of these. The mouse was distinguished by non-recognition of a major 14,000 MW antigen that was common to both L2 and L3/4 ES. The body fluid of adult Ascaris worms (ABF) was similarly characterized, and the antibody response to this was inferior to that of ES materials in discriminating between infected and non-infected animals. The major constituent of ABF was a 14,000 MW molecule homologous to that of larval ES products. The variability in host responsiveness to Ascaris antigens, revealed by the disparate responsiveness of three species of experimental animal, and the potential of ES for serodiagnosis, might prove of significance to immunological studies of ascariasis in man. Moreover, the heterogeneity of ES components questions the assertion of previous workers that the allergenic, IgE-potentiating, and protective activities of larval ES can be ascribed to one molecular species.