Human red blood cells (RBC) infected in vitro with Plasmodium falciparum were employed to prepare several types of antigens (sonicated, infected RBC and purified, sonicated merozoites and schizonts). These antigens, as well as control preparations derived from non-infected RBC, were used to coat plastic tubes, which were subsequently tested for capacity to bind anti-P. falciparum antibodies. Binding was detected by means of radio-iodinated staphylococcus protein A. Sera from patients with recent disease or patients who had a history of P. falciparum infection gave strong binding, while sera of normal individuals had only a low binding activity. Some of the antibodies in the positive sera were directed against RBC, since they could bind to tubes coated with normal RBC antigens and could be removed by absorption with RBC. The specificity of the P. falciparum antibodies was confirmed by inhibition tests: preparations derived from infected blood but not from normal blood inhibited the binding activity of the positive sera, to antigen coated tubes.