Transfection of an activated rat oncogene into NIH3T3 fibroblasts leads to transformation and induction of a metastatic phenotype. To identify genes whose activation might mediate these processes, we used a differential screening strategy. A 1.5-kb transcript is induced fiftyfold, constitutes 1% of ras transformed cell messenger RNA (mRNA) and is the most abundantly induced message in these cells. Our sequence data shows that it encodes murine cathepsin L, a potent collagenolytic and elastinolytic lysosomal enzyme. The murine clone was used to isolate human cathepsin L complementary DNA (cDNA) clones. The complete nucleotide and deduced amino acid sequences of human and murine preprocathepsin L are presented and compared to other papain family cysteine proteinases. Northern analysis shows that both human and murine cathepsin L probes hybridize to a 1.5-kb transcript in several tissues, but also to a 4-kb transcript in human kidney. These clones will facilitate studies of the structure, expression, and function of cathepsin L, including its unexpected upregulation in transformation.