A comparative study of serum antichymotrypsin, acid glycoprotein and antitrypsin levels was made in various human cancers. Antichymotrypsin (ACT) shows a non-specific elevation in metastatic cancer, and the response reflects tumor progression in a similar fashion to the other 2 acute phase reactant proteins. The main advantage of ACT as a marker appears to be its normal distribution in health, and lack of phenotypic variation.