The purpose of this research was to establish the influence of 3 j Cx cellulase applied per gram of COT concentrate mixture, fed in combination with a milk diet acidified by formic acid to the value of pH 4.6, on calf growth performance in one metabolism and two field experiments. In the metabolism experiment two groups of calves, with six animals in each, were fed acidified whole milk, which was diluted stage by stage till weaning at 60 days of age. The average live weight gain in the control at the end of the milk feeding period, i.e. from 14 to 60 days of age, was 29.90 kg. This corresponds to a daily live weight gain of 0.650 g. The total live weight gain of male calves in the experimental group was 29.30 kg, corresponding to a daily live weight gain of 0.638 g (Tab. I). Tab. II shows the average feed and nutrient intakes per kg live weight gain. The calves which received the enzyme supplement tend to have the higher feed conversion rate. During the forage feeding period, i.e. from 61 to 90 days of age, the average daily live weight gains were 1.10 kg and 0.980 kg in the control and experimental groups, respectively (Tab. III). The average live weight of 90 days old male calves was 107.70 kg and 103.90 kg in the control and experimental groups, respectively. The amount of consumed nutrients (digestible protein and starch units); in relation to the total feed intakes, is lower in the experimental groups, which proves the higher feed conversion rate (P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)