We have previously shown that the highly metastatic murine B16a melanoma expresses a high level of cathepsin B mRNA which is associated with three transcripts of 2.2, 4.0 and 5.0 kb, while in contrast only a single 2.2 kb cathepsin B RNA was detected in normal murine tissues. Using recombinant DNA techniques, cDNAs corresponding to these three transcripts have been isolated from a B16a melanoma cDNA library. Sequence analysis indicates that all three mRNA transcripts contain identical coding sequences for normal preprocathepsin B. However, the 4.0 and 5.0 kb transcripts contain unusually long extended 3' untranslated regions. These results suggest that the post-transcriptional processing pathway of the cathepsin B gene is modified in B16 melanomas. The results also indicate that the increased extracellular secretion of larger forms of cathepsin B by tumors is most likely due to post-translational mechanisms and does not involve alternative splicing or a coding mutation in the gene.