We investigated the anti-proliferative effects of recombinant human lysozyme (rHlys) on gastric cancer cell lines and normal human lung fibroblasts. Using conventional molecular cloning techniques we purified rHlys, which we incubated with cultured cells and measured the effects on cell proliferation and viability. At concentrations of 100 and 1000 microg/l, rHlys significantly inhibited the growth of human gastric cancer cell lines. In contrast, 10 and 50 microg/l of rHlys stimulated gastric cancer cell growth. None of the concentrations of rHlys affected cell viability. Only the highest concentration of rHlys (1000 microg/l) inhibited human lung fibroblast growth. Our results suggest that 100 microg/l is the optimum growth inhibiting concentration, which inhibited cancer cell growth but not normal cell growth. Our in vitro findings suggest that genetically engineered rHlys might inhibit human gastric cancer cell proliferation in vivo, so it might warrant further investigation as a potential novel anti-cancer agent.