Serum pepsinogen activity was determined in the blood serum of 126 clinically healthy cows by the method of Paynter. After slaughter the cows studied were classified into three groups according to established macroscopic changes to the mucous membrane of the abomasum: a group without changes, a group with ulcers and a group with other changes. The prevalence of the appearance of abomasal ulcers was 7%. In the group of cows with ulcer the average activity of pepsinogen was statistically significantly higher than in other two investigated groups (P<0.001). We found that the extent of changes to the mucous membrane of the abomasum had statistically the most significant influence on the activity of pepsinogen (P<0.001). The number of changes had smaller, but still statistically significant influence on the activity of pepsinogen (P<0.05). On the basis of a confidence level of 95%, we determined 5.0 U/L as the upper boundary of serum pepsinogen activity, which still does not signify major changes to the mucous membrane of the abomasum. The use of serum pepsinogen promised to provide a simple serum test to evaluate absence of subclinical abomasal ulcers in cows with known parasitological status.