We investigated the effects of human anti-sporozoite antibodies on the sporogonic development of Plasmodium falciparum in Anopheles stephensi. Equal volumes of washed human erythrocytes and human sera from 1) volunteers with protective immunity induced by immunization with irradiated P. falciparum sporozoites, 2) the same volunteers before immunization, or 3) Kenyans exposed to natural sporozoite transmission, were fed to cohorts of P. falciparum-infected A. stephensi on either day 5, 8, or 11 after infection. A fourth group of infected mosquitoes from the same cohort were not refed. In two experiments, the effects of anti-sporozoite antibodies were evaluated by determining the infection rates and parasite densities for oocysts and salivary gland sporozoites. There was no evidence that anti-sporozoite antibodies had any effect on the development or intensity of P. falciparum infection in A. stephensi. However, accelerated oocyst maturation was associated with mosquitoes taking a second blood meal, independent of serum source. Salivary gland sporozoites from mosquitoes that fed on immune human sera contained bound human IgG, which was detectable by indirect immunofluorescence assay. The infectivity and transmission potential of human IgG-coated sporozoites is unknown.