The serum activity of beta-glucuronidase (beta-gluc) has been presumed to indicate the disease activity in rheumatoid arthritis (RA). In 10 patients with RA the serum beta-gluc was repeatedly determined after the initiation of a treatment with cyclosporin for one year. A significant increase of beta-gluc was found after 8, 12 and 16 weeks compared to the values before treatment, while the concentration of the soluble interleukin 2-receptor decreased. The data reveal, that beta-gluc is not a useful indicator of the disease activity during cyclosporin treatment.