In endemic areas, malaria-infected mosquitoes may feed upon humans who possess antibodies against malaria sporozoites. Therefore, we examined the effect that ingested anti-sporozoite antibodies have upon Plasmodium falciparum sporogony within Anopheles stephensi mosquitoes. Anti-sporozoite antibodies (IgG) traversed the midgut into the hemocoel within 3 hr following ingestion and, depending upon the titer, persisted for 6-24 hr. When fed to infected A. stephensi at 12 days postinfection (p.i.), anti-sporozoite antibodies bound to sporozoites in the hemocoel, but not to sporozoites residing in the salivary glands of the same mosquitoes. Anti-sporozoite antibodies also bound to developing oocysts when fed to infected A. stephensi at 5 days p.i. Oocysts in mosquitoes that had been fed anti-sporozoite antibodies on Day 5 p.i. produced significantly more sporozoites than did oocysts in nonimmune-fed (Day 5 p.i.) mosquitoes. In addition, the sporozoites from Day 5 immune-fed mosquitoes were significantly more infective to cultured human hepatoma cells than were sporozoites from nonimmune-fed controls. Use of hetereologous immune feedings at Day 5 p.i. did not result in an enhanced production of sporozoites, suggesting that enhancement is related to the specificity of the antibody and is not merely a nutritional effect.