Synovial fluids from patients with rheumatoid arthritis (13 cases), ankylosing spondylitis (six), psoriatic arthritis (two), osteoarthritis (three) and rubella arthritis (four) contain interleukin-1 (Il-1) activity as measured in the CH3/He mouse thymocyte assay. That this activity is Il-1 was shown by: (i) the time course of thymocyte stimulation; (ii) failure of PHA blast cells to absorb out the activity; (iii) molecular weight of 15,000-20,000 daltons, and (iv) ability to stimulate prostaglandin E2 production from synovial cells. These results indicate that IL-1 is an important mediator in the inflammatory pathway of different types of joint disease.