We describe an approach to classifying a large number of clones expressing Plasmodium falciparum antigens in Escherichia coli by virtue of their differing reactivities with 100 human anti-malarial sera. Individual sera exhibited marked differences in the patterns of reactivity with these clones. These patterns led to the identification of sets of clones, here termed "serological families", which were shown to encode distinct P. falciparum antigens. A serological family was found to be composed of non-identical clones derived from portions of the same antigen. Using this approach six new P. falciparum antigens were identified. One of these is described in detail and is a 102 X 10(3) Mr antigen, predominantly of schizonts. Sequencing studies on four cDNA clones encoding parts of this antigen revealed blocks of hydrophilic dipeptide and tripeptide repeats and so the antigen has been termed the acidic basic repeat antigen (ABRA).