The heat-stable proteins known as S-antigens which are associated with Plasmodium falciparum malarial infections in man show considerable serological diversity. Usually, different S-antigens appear in separate malarial episodes in individuals exposed to reinfection. The antigens have a worldwide distribution in endemic areas but the restricted number and characteristic specificity of S-antigens recovered from experimentally infected Aotus monkeys suggest that these antigens might provide suitable markers for serotyping isolates of P. falciparum. Two kinds of evidence to support this idea are presented here. First, S-antigens which characterise an isolate of parasites are shown to retain their specificity over long periods of passage either in vivo or in vitro. Second, predictable mixtures of S-antigens can be recovered from cultures of P. falciparum after deliberately mixing isolates which each give rise to characteristic S-antigens.