Specific morphological and histochemical changes serve to define stages of differentiation during terminal myeloid maturation. The development of the hybridoma technology has allowed generation of monoclonal antibodies selectively reactive with antigenic determinants expressed in the hematopoietic system by myeloid cells at specific stages of differentiation. Here, the characterization by one of these antibodies i.e. GO35 (CD17) which shows myeloid specificity, was reported on blastic cells from 30% of acute lymphoid leukemia cases investigated (8/25). This monoclonal antibody may prove useful in the subclassification of atypical lymphoproliferative disorders including“hybrid leukemias”and serve as a possible prognostic factor for a therapy.