Various preparations of somatic Nematospiroides dubius antigens were emulsified in Freund's complete adjuvant and assayed in Quackenbush mice for efficacy as vaccines against homologous infection. Soluble and particulate fractions were given by two different routes. Only minimal protection was induced by vaccination with either soluble or particulate parasite fractions used independently when compared with the adjuvant control. Fewer and smaller worms were recovered from, and significantly fewer parasite eggs were voided by, mice which had been treated previously with both soluble somatic protein (15 micrograms N. dubius per mouse) by the intraperitoneal route and particulate antigen by subcutaneous injection. These mice showed higher antibody titres in comparison with other mice treated with either fraction alone. Further, treatment with both antigen fractions together appeared to exert a synergistic effect in comparison with either fraction administered alone.