Cysteine proteinases (CPs) are enzymes that belong to the papain superfamily, which are found in a number of organisms from prokaryotes to mammals. On the parasitic protozoan Leishmania, extensive studies have shown that CPs are involved in parasite survival, replication and the onset of disease, and have, therefore, been considered as attractive drugs and/or vaccine targets for the control of leishmaniasis. We have previously shown that cysteine proteinases, Type I (CPB) and Type II (CPA), in Leishmania major (L. major), delivered as recombinant proteins or in plasmid DNA, induce partial protection against infection with the parasite in BALB/c mice. We had shown that the level of protection was greater if a cocktail of cpa and cpb containing DNA constructs was used. Therefore, to reduce the costs associated with the production of these vaccine candidates, a construct was developed, whereby the cpa and cpb genes were fused together to give rise to a single hybrid protein. The genes were fused in tandem where the C-terminal extension (CTE), encoding region of CPB, was located at the 3' of the fused genes, and ultimately expressed in the bacterial expression construct pET-23a. The expression of the CPA/B hybrid protein (60 kDa) was verified using rabbit anti-CPA and anti-CPB antibodies by SDS-PAGE and immunoblotting. The protective potential of the CPA/B hybrid protein against the infection with Leishmania was then assessed in BALB/c mice. The animals were vaccinated with CPA/B, challenged with live L. major promastigotes, and the degree of protection was examined by measuring footpad lesion sizes. It was found that there was a delay in the expansion of lesions size compared to control groups. Furthermore, an immunological analysis of antibody isotypes, before and after infection, showed high levels of IgG2a compared to IgG1 (more than five-fold) in the CPA/B hybrid protein vaccinated group. In addition, a predominant Th1 immune response characterized by in vitro IFN-gamma production was observed, along with little, if any, IL-5 production. This finding indicates that the hybrid CPA/B is able to elicit a protective immune response against L. major in the mice model. In addition, 54% of individuals tested, who had recovered from cutaneous leishmaniasis, produced more than 50 pg/ml IFN-gamma, in response to the CPA/B hybrid protein in an in vitro assay, demonstrating the importance of cysteine proteinases as targets of immune response in humans.