SummaryFewer and smaller worms were recovered from, and significantly fewer parasite eggs were voided by, mice injected with mouse antiserum raised againstNippostrongylus brasiliensisorNematospiroides dubiusafter infection with the homologous nematode species compared with the control mice. The passive transfer of protective immunity was specific for the species which induced the antibody response in the donor mice. Passively immunized mice infected with the heterologous nematode species harboured the same number of worms, with the same epg output, as parasites in the control mice, but were stunted. Serological cross‐reactions were observed between the two donor antiserum pools andN. dubiusandN. brasiliensisantigens.