Serum alpha-thiol protease inhibitor (alpha-TPI) concentration was assayed by radial immunodiffusion in normal subjects, pregnant women, and in a wide variety of diseases. The normal concentration (448 (SD 75) mg/l) increased significantly (p less than 0.001) in pregnancy to 575 (89) mg/l, and in prostatic cancer treated by oestrogens to 666 (87) mg/l. Inconsistent changes were observed in inflammatory and malignant disease and in liver disease. A temporary pronounced fall of alpha-TPI was seen after burn injury, and a sustained fall after bone marrow transplantation. Crossed immunoelectrophoresis showed that the serum alpha-TPI occurred in two forms, with alpha 2 and alpha 1 electrophoretic mobilities. A heavy demand on this antiprotease may result in suppression of the alpha 1 form.