An enzyme-linked immunosorbent assay (ELISA) for circulating IgG mouse antibody to Plasmodium falciparum circumsporozoite (CS) protein was modified for use with human sera collected in an area of northern Zambia that was endemic for malaria and from individuals never exposed to malaria. Optimum sensitivity was achieved using Immulon 2 microtitration plates, boiled casein-Tween 20 blocking buffer, and by adding a solution of boiled casein (4 micrograms/ml) to the capture antigen diluent. The results for the detection of anti-CS IgG correlated well with those of sporozoite immunofluorescence antibody assays. Modification of the ELISA method permitted the simultaneous detection of anti-CS IgG and IgM antibody on a single serum sample in the same well of the microtitration plate and the detection of anti-CS IgG antibody in Kenyan dried whole-blood samples collected on filter-paper. The assay has been used to monitor human antibody levels in a phase-I malaria vaccine trial and in longitudinal studies of malaria transmission in Thailand and Kenya.