Sarcoidosis is characterised by changes in the lymphocyte subpopulations of the peripheral venous blood in the majority of patients. Thymus derived lymphocytes (T cells) are low whilst Bursa derived lymphocytes (B cells) are usually normal in number. The T cell lymphopaenia is best regarded as a "switching off" of available receptors as in vitro treatment with Levamisole restores this population to normal control values. A functional subset of T cells, T suppressor cells (T gamma) are activated, in sarcoidosis, in peripheral venous blood. Sarcoid sera is characterised by two inhibiting fractions; one, of low molecular weight (60 000 daltons) inhibits E rosette formation by normal T lymphocytes and may be a product of T suppressor cells or epithelioid/macrophage cells: the other is an immune complex which is capable of activating normal T suppressor cells and the serum immune complex concentration is positively correlated with suppressor cell activation in sarcoidosis.