An exported protein of the erythrocytic stages of the malaria parasite, Plasmodium falciparum, has epitope(s) in common with the surface of the sporozoite stage (1). Two cDNA clones encoding this protein, Ag5.1, have now been isolated and expressed in Escherichia coli. The coding sequence contains a region with strong homology to that of the circumsporozoite protein of P. falciparum. Other features of the sequence can be explained in terms of the observed behaviour of the protein in the parasite life cycle. The Ag5.1 can now be synthesised in bacteria in sufficient amounts to analyse the immune response to this protein.