Using the anti-TcR gamma/delta-1 monoclonal antibody and flow cytometry, we examined the number of T gamma delta cells in paired samples of peripheral blood and synovial fluid or tissue from 24 children with juvenile rheumatoid arthritis (JRA), five adult patients with JRA, and 14 patients with rheumatoid arthritis (RA). No significant difference was found in the synovial compartment T gamma delta values compared with the blood in JRA, adult JRA, or RA patients. Nor was any significant difference found in the peripheral blood or synovial compartment T gamma delta values in any of the three patient groups compared with the peripheral blood of normal controls. However, seven of the children with JRA had very high T gamma delta values in the synovial compartment while none of the normal children had high T gamma delta values in the blood (P = 0.02, Fisher's exact test). This may indicate a possible separate JRA patient group with high T gamma delta levels in the synovial compartment. In six JRA patients further analysed for T gamma delta subpopulations, a significant predominance of V delta 1+ cells was found in the synovial compartment compared with the corresponding peripheral blood samples (P less than 0.05, Wilcoxon's signed test) and with peripheral blood of child controls (P less than 0.05, Mann-Whitney U test). In these six patients, the T gamma delta-cell expression of the very early activation antigen CD69 were significantly higher (P less than 0.05, Wilcoxon's signed test) in the synovial compartment compared with the peripheral blood. Synovial T gamma delta cells expressing HLA-DR and interleukin 2 receptors could also be detected, in contrast to the peripheral blood in which no T gamma delta cells expressing these antigens could be found. These data suggest that the synovial T gamma delta cells had been activated in vivo.