Thirteen monoclonal antibodies have been raised against corticosteroid binding globulin (CBG). From four of those with highest affinity for the antigen, two were selected for development of a sandwich enzyme-linked immunoassay (ELISA). The sensitivity of the assay was such that 0.7 fmol CBG could be detected. Levels of the binding protein in men (740 +/- 67 nmol/l) and women (690 +/- 103 nmol/l) were not significantly different, while those found during the third trimester of pregnancy (1500 +/- 423 nmol/l) were approximately twice these levels. CBG denatured by heating to 60 degrees C could not be detected by the ELISA.