The different steps in the preparation of a certified reference material for gamma-glutamyltransferase are described. A preparation of pig kidney gamma-glutamyltransferase, partly hydrolyzed with papain, was prepared and lyophilized in a matrix containing bovine serum albumin. The between-ampoule variability of gamma-glutamyltransferase was estimated to be 0.6%. The predicted degradation at -20 degrees C is 0.00% per year. The certification procedure involved 15 participating laboratories. The certified gamma-glutamyltransferase catalytic concentration of the reconstituted material, using the IFCC proposed method, is 86.8 U/1 with a 0.95 confidence interval of +/- 2.1 U/1.