Mononuclear cell preparations from peripheral blood (PBL) and synovial fluid (SFL) of 27 Patients with rheumatoid diseases (15 patients with definite rheumatoid arthritis (RA), 10 with other inflammatory joint diseases (OJD), 1 with sarcoid arthritis (SA) and 1 with traumatic arthritis (TA) were examined for lymphocyte subpopulations determined by monoclonal antibodies of the OKT series and by the dot-like, acid alpha-naphthyl esterase staining (ANAE) activity. In patients with classic, active RA, blood T cells carrying the OKT8+ (suppressor/killer) phenotype were significantly reduced leading to an elevated OKT4/OKT8 ratio of 4.1 +/- 0.4 compared with 2.1 +/- 0.1 in healthy controls. In 10 patients with OJD this diminution of OKT8+ cells in peripheral blood was less pronounced or absent. As regards SFL subpopulations, patients with RA and OJD exhibited a similar distribution pattern with an elevation of OKT8+, Ia+ and ANAE negative cells and a similar OKT4/OKT8 ratio of 1.5 +/- 0.3 and 1.6 +/- 0.4, respectively. Similar results were also obtained in the only patient with TA, whereas the patient with SA and one RA patient with relapse after surgical synovectomy exhibited high OKT4/OKT8 ratios, both in synovial fluid and peripheral blood. Neither the OKT markers nor the dot-like ANAE staining pattern were significantly correlated to parameters of systemic or local disease activity as estimated by erythrocyte sedimentation rate and a local disease activity index.