A serum IgG response to Strongyloides papillosus infection was analysed in rabbits. IgG production was induced against 30-200 kDa larval somatic antigens and 21-160 kDa adult excretory/secretory (ES) antigens at 4 weeks postinfection. No immunoreactivity was detected between sera of infected animals and any proteins of larval ES and adult somatic antigens. Protection against larval challenge infection was investigated in rabbits immunized by primary infection and by intradermal inoculations of larval somatic and adult ES antigens. Primarily infected animals had 90% fewer or less worms from day 2 onwards after challenge, and lower faecal egg outputs after challenge, when compared with those in susceptible animals. Immunization with larval somatic antigens induced effective protection, showing 52.6-81.5% reductions in worm recovery from day 5 onwards after challenge, and lower faecal egg outputs after challenge. Systemic immunization with adult ES antigens failed to protect the animals against challenge. The possibility that resistance to S. papillosus reinfection is performed on the killing of migrating larvae in the early stages of infection is discussed. Systemic immune responses to larval somatic antigens might play an important role in the resistance.