Anabolic steroids are known to increase the plasma concentrations of certain plasma proteins. In four patients given treatment with danazol, an attenuated androgen, the concentrations of heparin cofactor II, Hageman factor (factor XII), protein C, and both free and total protein S increased significantly when tested 39 to 103 days after the start of therapy. The titers of these proteins in samples obtained 21 days to 5 years after therapy was discontinued were similar to those before treatment, except for total protein S, the titer of which remained elevated. No significant changes in the titers of C4b binding protein or plasma plasmin inhibitory activity were found.