Blood monocytes and synovial fluid and tissue macrophages were examined for their ability to produce interleukin-1 (IL-1) measured in a mouse thymocyte proliferation assay. Spontaneous production of IL-1 by monocytes from patients with rheumatoid arthritis (RA) or ankylosing spondylitis was higher than that by cells from normal subjects, patients with osteoarthritis or patients with RA treated with gold. IL-1 production in response to LPS stimulation was similar in all groups. Spontaneous IL-1 production by synovial fluid macrophages from patients with RA was similar to that of their monocytes, but the response to LPS was smaller. Synovial tissue macrophages produced little IL-1. Similar results were obtained in assays of fibroblast proliferation.