Human serum and urinary γ-glutamyltransferase (γ-GT) have been found to react differently towards the detergents Triton X-100 and sodium lauryl sulphate (SDS). Serum γ-GT was virtually unaffected by Triton X-100 at a concentration of 5% whereas urinary γ-GT was 10-15% activated under similar conditions. There was a 100-fold difference in the response of serum and urinary γ-GT to SDS. The enzyme activity in urine was completely destroyed by 0.02% SDS, whereas it required 2.0% to destroy the serum enzyme. These latter differences, however, were found to result from the environments of the serum and urinary enzyme rather than to intrinsic factors within the molecule. Extraction of serum γ-GT from normal individuals with «-butanol resulted in little or no loss of activity from the aqueous phase, whereas more than 30% activity was lost from normal urines. Extraction using various mixtures of butanol and di-/sopropyl ether (DIPE) produced largely similar results, except that 25% DIPE: 75% butanol mixture produced a marked loss of activity from serum. It is suggested that γ-GT activity in human body fluids may be dependent on the presence of a lipid fraction.