The role of peritoneal macrophages in a murine model of immunity to living hookworm third-stage larvae (L3) was investigated. Mice immunized orally with 500 L3 once every 2 weeks for three times were challenged intraperitoneally with 2000 L3 1 week after the final immunization. The challenged larvae were collected from the peritoneal cavity at intervals between 2 and 72 h and then examined by inverted light microscopy, scanning electron microscopy and transmission electron microscopy. Peritoneal cellular responses in non-immunized mice served as negative controls. The numbers of peritoneal macrophages in immunized mice were 6-7-fold higher than in non-immunized mice. In the peritoneal cavity of immunized mice, host macrophages adhered to the challenged L3 within 2 h and created a cocoon-like encasing which surrounded the parasite. Extensive damage to the L3 was observed which included swelling, collapse and deformation of the larval cuticle. Lysis and vacuolization of the parasite's internal structures were also observed. In contrast, no significant cellular adherence and damage were observed in L3 recovered from non-immunized mice. L3-specific antibody levels were also elevated in the peritoneum of immunized mice relative to non-immunized controls. These studies implicate macrophages as important effector cells in hookworm larval vaccine immunity.