An enzyme-linked immunosorbent assay for rapid detection of malarial antibodies and antigens was developed. Plasmodium falciparum antigen preparation, obtained from sonicated cultures of the parasite at a parasitemia of 10%-15%, was applied to cellulose filter discs in volumes of 0.1 microliter in 96-well microtiter plates. Antibodies were detected by successive incubations with: bovine serum albumin for blocking, tested serum at different dilutions, peroxidase-conjugated antihuman IgG, and the precipitable substrate 4-chloro-1-naphtol. Positive reactions appeared as blue dots on a white background which are easily read by eye. Pools of sera from patients with recent disease or from individuals with a history of malaria, contained antibodies detectable up to a dilution of 1:64,000. Negative results were obtained when normal RBC were used for dotting the filters. Normal sera showed no reaction at any antigen concentration. P. falciparum antigens were detected by their ability to inhibit the binding of antibody to the filters. RBC infected with P. falciparum in vitro can be detected at a level of 0.001% parasitemia. This report presents the feasibility of an assay for detecting malarial antibodies and antigens in blood samples which is easily applicable to field conditions.