cDNA encoding the highly conserved major surface glycoprotein, gp63, of Leishmania major was cloned, together with a signal sequence, into an eukaryotic expression vector, pCDNAI, which carries the human cytomegalovirus (CMV) promoter. This construct, pCMV/glycoprotein 63 (gp63), when injected into the skeletal muscle of BALB/c mice expressed sustained levels of gp63 in the muscle tissue for at least 40 days. Spleen and lymph node cells from the immunized mice produced significant amounts of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) but no detectable IL-4 when cultured with L. major antigens in vitro. The immunized mice also developed significant resistance against L. major infection compared to control mice injected with the empty plasmid. These results suggest that nucleic acid vaccine is effective against parasite infections.