Sera from a group of patients with juvenile chronic arthritis (JCA) were tested for soluble CD4 (sCD4). In most cases normal levels of the molecule were detected independent of disease activity. Similar results were obtained when sera from a population of adult rheumatoid arthritis (RA) patients were analyzed. Immunophenotypic studies of circulating mononuclear cells from seven JCA patients with active disease showed that T cells did not express activation markers. Finally, preliminary experiments showed that sCD4 levels were high in the synovial fluids from 3 RA patients as compared with paired serum determinations.