An assay procedure for ornithine carbamoyl transferase (OCT), which is known to be a liver-specific enzyme, was modified to be adaptable to bovine serum. The concentration of carbamoyl phosphate and the pH of the reaction reagent solution shifted to 60 mM and 7.2, respectively. These modifications contributed to the augmentation of assay sensitivity. The day-to-day reproducibility was 7.9% (coefficient variation) for serum with high OCT activity and 14.8% for normal bovine serum. The activity was stable for 3-4 months by the storage at -20 degrees C and at least 6 months at -80 degrees C. The OCT activity and other biochemical components including aspartate aminotransferase and gamma glutamyl transpeptidase were measured in sera from 164 dairy or raising cows reared at 7 farms. The normal level of OCT activity in sera from these cows ranged from 9.2 to 25.1 U/l (mean +/- 2SD). By comparison between the farms, the highest mean value of OCT activity was found in a farm where cows were suspected to have some kind of latent liver disease from the data of other biochemical parameters. We conclude that the OCT activity is a very useful diagnostic indicator of liver disease in cattle.