Fibrinogen is used in the diagnosis of inflammation as an acute phase protein. The research objective set by the authors of this study was to assess the applicability of fibrinogen concentration measurement in the blood plasma of the peak of lactating dairy cows through the evaluation of the reference values by using the functional PT-derived (Prothrombin Time-derived) method. Materials and methods: The study was carried out on 259 HF (Holstein-Friesian) and white-backed cows. The animals were clinically healthy. Fibrinogen concentration was determined by automated PT-derived method. Fibrinogen concentrations were calculated as the mean of duplicate samples. Samples with differences between duplicate results greater than 5% were rejected. Results: In the group of HF cows, the average fibrinogen concentration was 11.75 ± 4.80 g/L. In white-backed cows, it was 9.53 ± 4.79 g/L. At total of 76.01% of the results of the fibrinogen concentration in HF cows and 82.05% of the results obtained in the group of white-backed cows were within the ±1 SD (Standard Deviation) range. Conclusions: Based on our own research, the PT-derived method may be applied in order to determine the concentration of fibrinogen in cattle herds in animal health monitoring studies. An individual laboratory should focus more on verifying reference intervals established elsewhere.