The depressed cell-mediated immunity in rheumatoid arthritis was investigated in vivo by cutaneous hypersensitivity responses to seven antigens including tuberculin PPD, and in vitro by lymphocyte transformation to the latter antigen. In vivo 40% of rheumatoid patients were anergic compared to 2% of controls (P less than 0.001) with an associated reduction in sum score (5.9 +/- 6.5 vs 15.3 +/- 8.7, P less than 0.001). In vitro lymphocyte proliferation to PPD was also significantly depressed (P less than 0.001) and could not be reversed by indomethacin. A significant correlation between the in vivo sum scored (induration in mm) and in vitro thymidine incorporation (d/min) (r = 0.59, P less than 0.001) was found. In an attempt to overcome the depressed in vitro response the addition of a crude supernatant from a mixed lymphocyte reaction was found to return the PPD stimulated lymphocyte proliferation to the normal range. This effect was mimicked by purified IL-2 but not purified IL-1. The implications of this finding are are discussed.