R16HBsAg is an experimental recombinant malaria vaccine consisting of 16 repeats of a four amino acid sequence (Asn-Ala-Asn-Pro or NANP) of the circumsporozoite (CS) protein of Plasmodium falciparum expressed as a fusion protein with the recombinant hepatitis B virus surface antigen (HBsAg) produced by yeast cells. Twenty male volunteers were experimentally vaccinated with the product, as well as with two doses of the commercial recombinant HBsAg vaccine Engerix B (Smith Kline Beecham Biologicals, Rixensart, Belgium) at intervals during a period of 18 months. No serious side effects were observed. Circulating antibodies to recombinant CS antigen (R32tet32) developed in all volunteers and persisted in most cases over ten months. Anti-HBs antibody production was poor initially, but a single dose of the commercial hepatitis B vaccine was sufficient to elevate these titers to high levels in all but two volunteers.