Many proteins produced by blood stages of the malaria parasite Plasmodium falciparum are natural immunogens in man. As an approach to determining which of these are relevant to protective immunity we have constructed an expression library of P. falciparum cDNA sequences, cloned in Escherichia coli. The cDNA sequences were inserted into the beta-galactosidase gene of an ampicillin-resistant derivative of the temperature-sensitive lysogenic bacteriophage lambda gt11. About 5% of the resulting clones expressed P. falciparum sequences as polypeptides fused to beta-galactosidase. We have identified many clones that express P. falciparum antigens by immunological screening in situ with antibodies from immune human sera that inhibit P. falciparum growth in vitro. The antigen-positive clones contain P. falciparum cDNA sequences, as determined by hybridization. Some express polypeptides that are larger than beta-galactosidase and react both with antibodies to beta-galactosidase and with antibodies from humans immune to P. falciparum. The cloned P. falciparum antigens should facilitate new approaches to the identification of potential vaccine molecules.