In view of the fact that the ability of pig erythrocytes to use galactose will decrease with growing age, the galactokinase activity in pig erythrocytes of different age was determined by means of a spectrophotometrical method. A total of 5 pig fetuses (112th day of gestation) and 45 piglets at the age of 8 hours to 35 days belonging to different broods as well as 10 pigs for slaughter whose blood was collected after slaughtering out were included in the examination. In 5 piglets of one brood a progress examination was made during the suckling period. In the erythrocytes of all examined animals of different age groups the activity of the enzyme could be identified. There are great differences in the galactokinase activity of haemolysates of pig erythrocytes in different broods irrespective of their age. Therefore, an enzyme age dependence can only be recognized as a tendency from the comparison of the enzyme activities in erythrocytes of animals taken from different broods. In the erythrocyte haemolysate of 5 piglets of one brood the age dependence on galactokinase activity during the suckling period was determined in such a way that newborn piglets show a 30% higher enzyme activity than piglets at an age of 35 days. The enzyme activity in them, however, is still 1.6 times higher than in pigs for slaughtering. Thus, it may be concluded tht a postnatal decrease of galactokinase activity will occur in pig erythrocytes similarly to human erythrocytes. However, it is not responsible for a decrease of galactose utilization because no direct relations exist between both procedures. Parallel to glucose the decrease of galactose membrane permeability is assumed to play the decisive role here.