Tumor resistance to the glycopeptide anticancer drug bleomycin (BLM) has been suggested to involve metabolic inactivation by BLM hydrolase. Direct evidence for this hypothesis is lacking due to difficulties in obtaining full-length BLM hydrolase cDNA from mammalian cells. In the present investigation, we used the yeast cysteine proteinase gene ycp1, a homologue of the mammalian BLM hydrolase gene, to provide direct evidence of the importance of BLM metabolism in BLM resistance. Transfection of ycp1 into NIH 3T3 cells induced resistance of these cells to BLM. The ycp1-transfected cells also metabolized BLM A2 to its inactive metabolite deamido-BLM A2 to a much greater extent. The ycp1-induced BLM resistance was completely reversed by the cysteine proteinase inhibitor E-64, a known inhibitor of BLM hydrolase. Transfection of NIH 3T3 cells with the plasmid pUT533-Sh ble, a bacterial BLM resistance gene that encodes a 14-kDa protein that does not metabolize BLM, also induced BLM resistance, but this resistance was not overcome by E-64. The results demonstrate that increased BLM hydrolase activity in NIH 3T3 cells causes BLM resistance and that inhibition of BLM metabolism sensitizes these cells to BLM. Thus, the molecular approach described in the present study directly implicates BLM hydrolase in BLM resistance.