The responsiveness to mitogens of the in vitro cultural peripheral blood lymphocytes from 60 patients with rheumatoid arthritis (RA) and from controls was evaluated in terms of H3 TdR uptake. The size of the response was tested for a correlation with the percentages of T and B lymphocytes, determined by E and EAC rosette test, as well as with the various alterations of the clinical and biological parameters. In the RA cultures an elevated level of spontaneous H3 TdR uptake was found, which could be an in vitro carry-over of an activation in vivo. No significant statistical differences were found between rheumatic and control groups as regards: 1 degrees--the H3 TdR uptake induced both by PHA and SLO; 2 degrees--the proportion of the circulating T and B lymphocytes. The only positive correlation evidenced by this study in RA patients was that between the proportion of B lymphocytes and the gammaglobulin level in the plasma.