The migration of young adult-stage Heligmosomoides polygyrus from beneath the muscularis mucosa to the lumen of the intestine was monitored to compare the rate of development and maturation of larvae in normal and previously infected mice. The development of surviving larvae was significantly retarded in mice that had experienced one or more previous infections and the adult worms arising from a challenge infection were stunted and appeared anaemic. Identical effects were observed with worms recovered from mice that had been injected with immune mouse serum at the time of challenge, and the magnitude of these effects was related to the amount of serum given. Larval maturation was also retarded in mice immunized with larval excretory/secretory (ES) antigens, even though the antibody response was poor due to the very small (submicrogram) amounts of antigen available for injection. In contrast, larvae developed at a normal rate in mice that had been hyperimmunized with killed exsheathed larvae. These mice had serum antibody titres against both "internal" and cuticular antigens similar to those of highly immune (4x-infected) mice, but they had no detectable antibody against ES antigens. The results indicate that the growth and development in vivo of H. polygyrus larvae are retarded by antibodies specific for larval ES antigens. Stunting is permanent, with female worms being affected more severely than males and egg output per worm correspondingly reduced.