To investigate the cellular mechanisms underlying clinical anergy, studies were done to determine whether the T cells of anergic patients could be sensitized in vitro. Ten normal males and 20 male patients with peripheral vascular disease were skin-tested with seven antigens and their peripheral blood mononuclear leukocytes (PBL) isolated. The mononuclear cells were stimulated in vitro with keyhole limpet hemocyanin (KLH) for 10 days in serum-free medium (primary culture) and then restimulated with KLH for three days (secondary culture). None of the controls or patients had been sensitized previously with this antigen. Of the 30 different cell preparations we tried to sensitize, 21 responded with a stimulation index (S.I.) of 1.5 or greater after being stimulated with KLH in the secondary cultures. There was no correlation between the concentration of KLH in the primary culture, the concentration of KLH in the secondary culture, and the S.I. at the end of the secondary culture. Cell preparations from all subjects were stimulated by PHA in both the primary and secondary cultures. Whether or not a subject's PBL could be sensitized in vitro did not correlate with either the subject's response to the intradermal injection of antigens or whether the subject was a control or a patient.