Cell mediated immune functions were studied in 17 patients with rheumatoid arthritis (RA) at times off all drug therapy, and were correlated with clinical findings. Cellular immunity was evaluated by (1) skin testing to antigens and (2) measuring peripheral blood lymphocyte (PBL) and effusion lymphocyte tritiated thymidine (3H-TdR) uptake in in vitro cultures containing 20 percent autologous or 20 percent AB plasma and various concentrations of mitogens or antigens. Cutaneous and in vitro reactivity were decreased in RA patients: (1) RA PBL spontaneously incorporated more 3H-TdR than normals; (2) 3H-TdR uptake by RA PBL, cultured in AB plasma, in response to mitogens and antigens was markedly reduced when compared with normals; (3) RA PBL responses were further diminished when cells were cultured in autologous plasmas; (4) effusion lymphocytes similarly often had high unstimulated 3H-TdR uptakes and poor responses to stimulation; and (5) decreased proliferative responses of RA PBL (in AB plasma) correlated with class and stage III-IV, increasing age, number of tender or swollen joints, rheumatoid factor titer, total protein, decreased grip strength, and poor skin test reactivity.