Abstract.Among the various immune abnormalities which characterize active sarcoidosis, a low proliferative response of peripheral blood lymphocytes to mitogenic lectins has long been observed. Since membrane‐associated G‐proteins are very likely to be crucial elements in lectin signal transduction, we investigated the binding of 5′‐guanylylimidodiphosphate (GppNHp), a non hydrolyzable GTP analogue, to blood total lymphocyte membranes and to blood T‐lymphocyte membranes from patients with active sarcoidosis, and from healthy control subjects. GppNHp binding was markedly decreased in peripheral cells from patients with sarcoidosis as compared to controls, suggesting the occurence of a detect at the level of G‐protein(s). A further characterization of G‐proteins in these cells by means of ADP‐riboselabelling in the presence of bacterial toxins brought forward a significant decrease in the labelling of a 40 kDa protein, the major pertussis toxin substrate, in membranes from sarcoid patients, while the labelling of the major 44 kDa cholera toxin substrate proved to be unchanged with respect to control membranes. It is hypothesized that, in sarcoid lymphocytes, a detect in the negative control of adenylate cyclase mediated by the inhibitory G‐protein Gi, prevents the lowering of cAMP necessary to normal mitogenic response of blood lymphocytes to stimulation. cAMP degradation by the specialized enzyme phosphodiesterase constitutes another critical step in the control of cAMP levels. Both CAMP and cGMP phosphodiesterase activities were found decreased in blood total lymphocyte preparations from sarcoid patients. With purified T‐cells, although the mean cAMP and cGMP phosphodiesterase activities from sarcoid patients were found more markedly decreased with respect to healthy donors, only the decrease in cGMP phosphodiesterase was found statistically significant. The role these detects in cyclic nucleotide degradation potentially play in the disturbance of blood lymphocytes response associated with sarc