The carbohydrate segment of the excreted factor of culture cells from L. tropica and L. donovani was shown to interact with the adenylate cyclase from hamster liver. At a concentration of 1 mg/ml the excreted factor inhibited the stimulated activity of the mammalian enzyme by about 50%, independent of the activation by glucagon, 5'-guanylylimidodiphosphate and forskolin, respectively. In contrast to the adenylate cyclase system from hamster liver the enzyme from L. tropica-culture cells was not stimulated by addition of glucagon, 5'-guanylylimidodiphosphate and forskolin. In addition, the activity of the adenylate cyclase from L. tropica was not affected by addition of its endogenous excreted factor.