UNLABELLED: The merozoite surface protein-3 long synthetic peptide (MSP3-LSP) comprises the amino acid sequence 186-276 of the Plasmodium falciparum protein MSP3. It is currently in development as an erythrocytic stage (blood stage) malaria vaccine candidate. We report here the first data on the safety, reactogenicity and immunogenicity of three doses of MSP3-LSP, adjuvanted with aluminium hydroxide, in healthy male adults living in a malaria endemic area. METHODS: A phase 1b single-blind controlled trial was performed in the village of Balonghin in Burkina Faso. Thirty male volunteers aged 18-40 years were randomised to receive either three doses of 30 microg MSP3-LSP or 0.5 ml of tetanus toxoid vaccine. The second and third vaccine doses were given 28 and 112 days after the first dose. We followed participants for 1 year. RESULTS: There were no serious adverse events in either vaccine group. In both groups participants reported local reactions at the site of injection when compared to an earlier trial in European volunteers. Only one systemic adverse event (tachycardia) was identified which occurred immediately after the first vaccination in one individual receiving MSP3-LSP. No clinically significant biological abnormalities following vaccination were observed. Humoral immune responses (IgG, IgG subclasses, IgM) to MSP3-LSP peptide were similar in the two groups following vaccination. Some cell-mediated immune responses appeared to differ between the two vaccine groups. After the second dose of MSP3-LSP, there appeared to be a marked increase in the lymphocyte proliferation index and IFN-gamma in response to stimulation with MSP3-LSP. CONCLUSION: These data suggest that three doses of 30 microg MSP3-LSP when administered subcutaneously on days 0, 28 and 112 are well-tolerated by adult males previously exposed to natural P. falciparum infection. They also suggest that MSP3-LSP is able to stimulate an enhanced cell-mediated immune response in individuals with some degree of preexisting immunity.