Four commercially available kinetic photometric tests for the determination of gamma-glutamyl transferase were compared. Test A is based on the substrate L-gamma-glutamyl-4-nitroanilide; test B uses this same substrate dissolved in hydrochloric acid. C1 and C2 are tests from different suppliers; both use the substrate L-gamma-glutamyl-3-carboxy-4-nitroanilide, and all other constituents are the same as quoted by the suppliers. Parallel determinations were performed on 1017 native sera and 12 control sera. Compared with values from test A, two of the three other tests showed statistically significant differences in the diagnostically important range 10 to 50 U/l. Significant differences were also observed between the results from the apparently identical tests, C1, and C2. The values obtained for control sera showed a dependence on the test procedure and/or composition.