Bone marrow blast cells of 174 child and 188 adult patients with AML were examined and characterized in terms of their FAB type, immunological phenotype (102 children, 123 adults) and karyotype (69 children, 95 adults). The incidence of FAB variants of AML proved similar in children and adults. In patients under 15 and over 60, peroxidase activity in myeloblasts was lower than in middle-aged patients. Similar rates of HLA-Dr. Thy-1, CD11a, T-cell antigens, CD19, Gly-A and Eb antigens were found in cells of child and adult patients. The frequency of CD11b, CD38 and CD10 antigen expression on blast cells was higher in children than in adults. Abnormal blast karyotype was noted in 81.8% of children and 73.7% of adults. Translocation (8;21) was usually found in cases of M2 type (82%), significantly more frequently in children. predominantly in the group aged 6-10. t(15;17) was detected in all age groups only in M3 type of cells (86%). t(9;22) occurred more frequently in adults than in children; t(11q23) incidence rates were somewhat higher in children than in adults. Three cases of AML in children are described with deletion of chromosome 5 in their leukaemic cells. The data obtained indicate different biological characteristics of blast cells in children and adults. It is likely that haemopoietic cell involvement in children under 2 years and adult patients over 60 occurs at earlier stages than in middle-aged patients.