Human palatine tonsil lymphocytes, when compared with peripheral blood lymphocytes (PBL), were in an activated state, even though there was no in vitro stimulation. When these tonsil lymphocytes were cultured in the absence of serum and polyclonal mitogens or antigens, the supernatant fluid often inhibited the proliferative response of target PBL to concanavalin A. The extent of this suppression ranged from 22% to 84%, and target cell viability was 90% or greater. There was no evidence for the presence of immunoglobulins or α2-macroglobulin in whole supernatant fluids. The suppressor was partially denatured at 80°C and was rendered completely inactive upon exposure to 100°C for 5 min. It was trypsin-sensitive, and had an apparent molecular weight of 100,000 or greater. The protein adhered strongly to DE-52, and the most active material was eluted with 0.4–0.6 M NaCl. Only one component was detected in this fraction by polyacrylamide gel electrophoresis. The suppressor had an isoelectric point of 5.0 ± 0.6.