The respective investigations were carried out in the central rearing station of bulls at Mrozowo on 92 animals aging 1-12 months. Every month blood samples were taken from bulls and the level of hemoglobin, erythrocytes, leucocytes, hematocrit, total protein and its fractions, urea, α-amine nitrogen, AspAT, ALAT, AP, glucose, cholesterol and bilirubin was determined. The results were subjected to the statistical analysis. It has been found that along with the growth of bulls increased in the blood serum the total protein level due to an increase of γ-globulin fractions. The activity increased since the age of 5 months and then it gradually decreased. The cholesterol level increased since the age of 4 months. Among the blood indices investigated the stablest values in subsequent month of life of the bulls showed erythrocytes, hematocrit and albumins.