According to recent investigations neopterin (a pyrazinopyrimidine derivative) is a biochemical marker that reflects the activity of the proinflammatory immunocellular system of the synovial tissue in rheumatoid arthritis (RA). Interferon gamma, derived from antigen activated T lymphocytes, stimulates macrophages to synthesise and release neopterin into the culture supernatant in vitro. To extend this in vitro model to a clinical level a sensitive new radioimmunoassay technique was used to measure neopterin concentrations in the synovial fluid (SF) of 17 patients with active RA, nine with osteoarthritis, and six with acute gout, and in that of 12 controls undergoing meniscectomy. The SF neopterin concentrations were significantly higher in patients with RA than in the other groups of patients, particularly the controls. Multivariant analysis showed that SF neopterin concentrations correspond better with the systemic inflammatory activity of RA than with the local disease activity of the knee joints. Thus the study strengthens the hypothesis that neopterin reflects the essential role of the activated immunocellular reaction in the pathogenesis of RA.