Stromelysin and stromelysin 2, closely related members of the metalloproteinase gene family degrade many non-collagenous components of the extracellular matrix and may play a role in the activation of latent procollagenase. Because we use monolayer cultures of rabbit and human fibroblasts as model systems to study these enzymes, we compared their expression in fibroblasts from both species. Rabbit stromelysin purified from fibroblast culture medium often appears as a protein doublet, while human stromelysin is a single protein band. Hybrid selection with a cDNA clone for rabbit stromelysin and in vitro translation of mRNA from rabbit fibroblasts stimulated with phorbol myristate acetate (PMA) reveals two translation products, Mr54 and 56KD, as measured by SDS polyacrylamide gel electrophoresis. In vitro transcription and translation of a 1.8 kb cDNA for rabbit stromelysin gives a single protein product, preprostromelysin, MR 56KD. We do not yet know whether the rabbit doublet represents two distinct gene products or whether it results from posttranscriptional/posttranslational processing of a single transcript or protein. To study human stromelysin, we cloned a cDNA from a rheumatoid synovial cell cDNA library and we used it to isolate genes for stromelysin and a related gene, stromelysin-2. Both genes are contained on approximately 14 kilobase pairs of DNA. With an exon containing fragment of the human stromelysin-2 genomic clone as a specific probe in Northern blot analysis, we demonstrate the differential expression of stromelysin and stromelysin 2 in rheumatoid synovial cells, human foreskin fibroblasts, and rabbit synovial fibroblasts. Chimeric constructs containing 302 bp of the human stromelysin promoter DNA linked to the bacterial gene chloramphenicol acetyl transferase (CAT) can be induced by PMA, epidermal growth factor (EGF) and interleukin-1 beta (IL-1 beta). Since the genes for stromelysin and stromelysin 2 are so conserved and since mechanisms regulating their expression appear to be distinctive, identification of these mechanisms in both rabbits and humans will increase our understanding of the relative role of these enzymes in normal and disease processes.