Stromelysin is a collagenase-related connective-tissue-degrading metalloproteinase. We have detected RNAs capable of hybridizing to a rat stromelysin cDNA in 11 of 69 human tumours tested. Molecular cloning of cDNAs to these RNAs has identified them as a mixture of stromelysin RNA and a transcript of a hitherto-undescribed related human gene, the stromelysin-2 gene. We have also isolated cDNAs corresponding to a more distantly related new human gene, the pump-1 gene. A comparison of the cDNA-derived amino acid sequences of stromelysin-2 and pump-1 with the known sequences of stromelysin and collagenase reveals significant similarities, with conservation of sequence motifs believed to have functional importance in metalloproteinase action. We conclude that the collagenase gene family in humans consists of at least four members, and speculate that expression of these genes plays a role in cancer.