The wide hepatic distribution, high cytosolic concentration, and short in vivo plasma half-life of serum alpha-glutathione s-transferase are properties which may make monitoring this enzyme more clinically useful than conventional biochemical liver function tests as a marker of hepatocellular damage associated with acute liver allograft rejection. In a prospective longitudinal study of 58 liver transplants in 45 patients, serum alpha-glutathione S-transferase concentrations rose significantly more consistently and more rapidly than conventional liver function tests in association with acute rejection. However, a rise in alpha-glutathione S-transferase was less specific for rejection than conventional liver function tests although none of the tests had a positive predictive value for rejection of greater than 32%. Compatible with the particularly short in vivo plasma half-life of this enzyme, alpha-glutathione S-transferase concentrations fell to or toward normal more rapidly than conventional liver function test measurements following uncomplicated transplantation as well as during high-dose steroid treatment of rejection. This may be valuable, both in improving the resolution of biochemical changes associated with early rejection episodes and in determining when treatment of rejection has been successful. Further studies are warranted, however, to assess whether the fall in GST during rejection treatment does genuinely reflect the histological resolution of rejection.