Human circumsporozoite (CS) antibodies to Plasmodium falciparum were detected in blood meals from 45.0% of 1,547 field-collected Anopheles gambiae Giles sensu lato and Anopheles funestus Giles from western Kenya. Possible effects on malaria infections within the Anopheles host were investigated. Circumsporozoite antibodies were detected in blood meals up to 36 h after feeding. Antibodies crossing the midgut were detected experimentally in hemolymph from 4 to 36 h after feeding; human IgG also was present in hemolymph from fully gravid field-collected Anopheles. Ingestion of high-titer human CS antibodies or 2A10 monoclonal antibody to P. falciparum sporozoites by P. falciparum-infected An. gambiae, 10 d after feeding on an infected human, had no effect on oöcyst maturation, sporozoite rates, or sporozoite loads. Contact between CS antibodies and sporozoites in the hemocoel did not block sporozoite invasion of salivary glands. Human IgG antibodies were detected by an indirect fluorescent antibody technique on salivary gland sporozoites from 83.3% of 114 field-collected Anopheles. In 65.4% of 26 infections, antibodies persisted on sporozoites for at least three days. Thus, a high proportion of naturally infected An. gambiae s.l. and An. funestus in western Kenya transmit sporozoites that are bound with human IgG acquired during previous blood meals. The infectivity of such sporozoites needs to be determined in relation to natural transmission and to the potential use of malaria sporozoite vaccines.