Monolayers of rabbit synovial fibroblasts treated experimentally with phorbol myristate acetate produce large amounts of collagenase and prostaglandin E2 and have been a suitable experimental model for the proliferative/destructive lesion of rheumatoid arthritis. We used X-irradiation to prolong the in vitro lifespan of these cells so that cloned populations could be studied. By a number of criteria, X-irradiation did not alter the cells to make them unrepresentative of synovial fibroblasts. With limiting dilution techniques, we simultaneously isolated three clones. These clones were shown to have different growth rates and to produce different amounts of collagenase and prostaglandin E2. Rates of protein synthesis, measured by incorporation of 3H-leucine, were similar for all three clones. Our data support the concept that particular populations of synovial cells may contribute selectively to the joint destruction seen in rheumatoid disease.