The response of peripheral lymphocytes of patients with Kveim positive sarcoidosis to a Chase-Siltzbach type-I (spleen J) suspension, normal spleen suspension, and phytohæmagglutinin (P.H.A.) was assayed in-vitro by 14C-leucine incorporation (nineteen patients), release of migration inhibitory factor (M.I.F.) (seventeen patients), and direct migration inhibition (six patients). No specific response to Kveim suspension in a wide range of doses was detected by these assays. All patients showed diminished responsiveness to P.H.A. when compared with controls. A high spontaneous release in vitro of M.I.F. was found for the lymphocytes of patients with sarcoidosis and could not be augmented by addition of Kveim antigen.