The parasitic stages of Cooperia punctata were cultivated in vitro, and the incubation fluid from these stages containing exoantigen (ExoAg; metabolites, secretions, excretions) was diafiltered (retaining material of approximate molecular weight greater than 10,000), concentrated, and lyophilized. Using serum of calves immune to C punctata, ExoAg activity was verified by agar gel immunodiffusion. The ExoAg was combined with adjuvants and was inoculated into calves. Results of preliminary experiments indicated that the ExoAg had potent biological activity capable of inducing severe respiratory distress and death. Subsequently, however, improved inoculation procedures and combination of ExoAg with aluminum hydroxide gel or Freund complete adjuvant consistently prevented this reaction in 25 calves. In helminth-free calves, the ExoAg induced marked eosinophilia, which generally was greater during the ExoAg-exposure period than after challenge exposure with infective larvae. Of 27 calves exposed to the various ExoAg inoculation schedules, 12 had nematode reductions greater than 80%, 10 had less than 16% reduction, and 5 had reductions between 39 and 72%. This polarization of the nematode recovery data generally thwarted statistical difference in overall consideration of all ExoAg inoculation schedules. Additional manifestations of resistance, ie, prolonged prepatent period, reduced patent period, reduced number or lack of nematode eggs passed in feces, and reduced nematode length, were observed in calves of certain ExoAg inoculation schedules. In vitro blastogenic responses of blood lymphocytes was observed in calves exposed to ExoAg. Quantitatively, this cell-mediated immune response to ExoAg was as great as or greater than that after subsequent challenge exposure with infective larvae. Intradermal inoculation of ExoAg into a calf resistant to C punctata induced delayed-type hypersensitivity. However, this reaction to ExoAg was not observed in nonimmunized, helminth-free calves used as controls. Humoral antibodies against ExoAg were detected by an agar gel immunodiffusion test. Calves exposed to ExoAg always seroconverted and usually remained seropositive until necropsy. There were no false-positive results; however, ExoAg did not consistently coprecipitate (by agar gel immunodiffusion) with serum from infected calves that were not exposed to ExoAg.