Sera from 10 of 14 patients with sarcoidosis reduced the in vitro transformation of normal lymphocytes to Candida and mumps antigens. The response of lymphocytes to phytohemagglutinin was also diminished by 7 of 14 sera. The inhibitory activity of sarcoidosis patients sera on normal lymphocytes did not correlate well with the results of in vivo studies of cellular immunity in the serum donors. The impaired cell-mediated immunity in patients with sarcoidosis may be, in part, related to lymphocyte inhibitors in their sera, however, these studies suggest that sarcoidosis patients may have an additional unidentified immune defect.