The corticosteroid-binding globulin (CBG) is a plasma protein which is present both in liver, where it is mainly synthesized, and in cells of different target tissues for glucocorticoids. Using monospecific antibodies raised against human CGB, we could demonstrate the antigenic identity of the protein in human bronchoalveolar fluid. We found that the bronchoalveolar fluid/serum concentration ratio of CBG was similar to that of albumin. Since albumin is not synthesized in pulmonary cells, it was concluded that, in healthy human, CBG enters bronchoalveolar fluid by diffusion through alveolar cells. It is suggested that the expression of the CBG gene in pulmonary cells could occur during the pathological state.