In human malarial infections, the immunological importance of modified host cell components and of parasite antigens that may be released on spontaneous rupture of infected erythrocytes in vivo is not known. It is difficult, however, to analyse antigens extracted from entire parasitized red cells because of contamination with normal red cell constituents, notably haemoglobin. To overcome this problem, a simple chromatographic procedure has been developed that concentrates antigens and radiolabelled parasite products in a separate fraction from haemoglobin. Details are also given of investigations of two different kinds of antigen associated with Plasmodium falciparum. One antigen known to be immunogenic in man is shown to be associated with membranes. The other antigen is poorly immunogenic in man and other animals and is shown to be a soluble, rather stable, acidic protein.