Rheumatoid arthritis (RA) synovial fluid (SF)-T lymphocytes appear relatively inactive in situ and respond only weakly to diverse stimuli ex vivo. To characterize the molecular defects underlying this hyporesponsiveness we analyzed the expression level of several proteins involved in TCR-proximal signal transduction. As compared to peripheral blood (PB)-T lymphocytes, SF-T cells from some (but not all) of the patients analyzed expressed lower levels of TCRαβ, CD3ε, TCRζ, p56^lck and LAT, while p59^fyn, phospholipase C-γ1 and ZAP-70 expression was unaltered. Semi-quantitative analysis of T cells from several patients revealed that the degree of TCRζ chain and p56^lck modulation correlated statistically significantly with the level of SF-T cell hyporesponsiveness. The differential reactivity of p56^lck specific monoclonal and polyclonal antibodies in SF-T but not PB-T lymphocytes indicated that p56^lck modulation consists of a conformational change rather than loss of expression. Our results indicate that multiple signaling molecules can be modulated in RA SF-T cells and show for the first time a direct quantitative correlation between T cell hyporesponsiveness and modulation of TCRζ and of p56^lck, a critical protein tyrosine kinase required for T cell activation.