Leukemic cells as well as normal blood cells possess several antigens on their cell surfaces. By detecting these cell-specific molecules using monoclonal antibodies and flow cytometry, these immunological methods can be applied to both clinical and basic tumor studies. The analysis of cell surface markers has been applied in the diagnosis and the management of hematological disorders such as leukemias and lymphomas and it has become to be utilized as the routine work in many laboratories. Such analysis has been particularly useful in lymphocytic leukemias due to the ability of this method to classify lymphocyte subtypes, as well as specify their developmental origin. Importance is that, with this information, it is possible to determine expected clinical prognosis, to recommend treatment options and to judge treatment efficacy.