377 untreated acute leukaemia patients were categorized according to FAB and cytochemical criterials and simultaneously phenotyped with the use of 6-21 monoclonal antibodies (MoAb) of VI series (W. Knapp, Vienna). The leukaemia phenotype was compared with the patients outcome after treatment. In adult ANLL patients a positive relationships was proved statistically between the expression of the CD 15 cell differentiation antigen on leukaemic blasts and the CR rate (p less than 0.01, chi 2 test). Also a comparison of the Kaplan-Meier survival curves revealed that the CD 15 positive group of ANLL patients has a better outcome than the CD 15 negative one (p less than 0.01, by Wilcoxon and Log-rank tests). Thus, examination of cell differentiation antigens could be a useful addition to existing risk assignment in acute leukaemia.