BACKGROUND Arginase (EC 3.5.3.1) is one of the essential enzymes in the terminal stages of the urea cycle in the liver which participates in the elimination of ammonia from the human body [1, 7]. Except in liver tissue arginase is also present in many human tissues and in the circulating blood cells, especially in erythrocytes and leukocytes. Arginase splits arginase to urea and ornithine that serve for biosynthesis of amino acid proline, glutamic acid and biosynthesis of polyamines-spermine, spermidine and putrescine. Arginase activity is high during the mitotic cycle, with the function in phase S of the cell cycle. The aim of our study was to assess the arginase activity in the blood of children with some haematologic diseases. METHODS We examined the arginase activity in blood plasma and erythrocytes of children who suffer from some haematological disorders (27 patients) and in healthy children (control group-15 subjects). The enzyme activity was measured with spectrophotometric method on the basis of the determination of the amount of liberated ornithine from substrate-arginine [3]. RESULTS The obtained results suggest that arginase activity was much higher in the blood of ill children (Table 1 and Figure 1). In the control group of children (total 15) plasma arginase activity was in the range of 0 to 20 U/L x = 0.86 U/L), and enzyme activity in erythrocytes was 1.62-3.98 U/g Hb (x = 2.81 U/g Hb). Erythrocytes enzyme activity and plasma enzyme activity were in ranges of 4.03-5.26 U/L with the mean value of x = 4.56 U/L, and arginase activity in erythrocytes was in ranges of 9.38-14.16 U/g Hb, with mean value x = 11.34 U/g Hb, respectively. Arginase activity in erythrocytes was also significantly higher in children with non-spherocytic haemolytic anaemia (9 children) and was in ranges of 5.33-9.58 U/g Hb (x = 7.29 U/g Hb), with the relatively low values in plasma, 0-4.14 U/L (x = 1.75 U/L). In children with sideropenic anaemia (total number-11) arginase activity in erythrocytes was also very significantly increased with the range between 2.86 and 14.16 U/g Hb (x = 5.54 U/g Hb) while the plasma enzyme activity was relatively low, with the values in range of 0-4.98 U/L (x = 1.41 U/L); in myelodysplastic syndrome (4 pts) arginase activity in plasma was very low (0-0.71 U/L; x = 0.26 U/L) with higher values of arginase activity in erythrocytes (4.22-5.89 U/g Hb; x = 4.94 U/g Hb). CONCLUSIONS We have concluded that the enzyme activity was the highest in erythrocyte haemolysates of patients with spherocytosis, non-sherocytic haemolytic anaemia; it was also high, but in a smaller degree, in erythrocytes of children with sideropenic anaemia and myelodysplastic syndrome; arginase activity in plasma of these children was higher in comparison with enzyme activity in plasma and erythrocytes of healthy children. Our results are in agreement with data from literature where it is stated that the younger erythrocytes have the highest arginase activity than the mature erythrocytes [4]. CONCLUSIONS The measurement of arginase activity in plasma and erythrocytes is a good diagnostic indicator for the presence of young erythrocytes and reticulocytes in the circulating blood as is the good sign for the detection of haemolytic processes.