A panel of 14 monoclonal antibodies (McAb) against hematopoietic cell surface antigens was applied on mononuclear blood or bone marrow cells from 40 cases of acute leukemia in order to compare immunoenzymatic staining (IE) (alkaline phosphatase) of fixed cells with immunofluorescence staining (IF) of unfixed suspended cells. According to the immunological results, 25 cases were phenotyped as ALL and 15 cases as AML. Cases with blast crisis secondary to chronic myelogenous leukemia (CML-BC) were not represented in this series. In all ALL cases the two methods gave an identical antigenic distribution. In 20 our of 21 cases of non-T-cell ALL, a B-cell progenitor origin was demonstrated by a positive staining reaction with the anti-CD19 McAb AB1 or HD37, and in 10 cases additionally with the anti-CD20 McAb B1 or 1F5. In contrast to the results obtained with IF, IE revealed a poor preservation of the AB1 epitope on CD19, whereas the HD37 epitope was equally well demonstrated by both methods. In 15 cases of AML the distribution of positive versus negative cells with IE or IF was identical for all McAb except J5 (anti-CALLA) (CD10) and B1 (CD20). Thus, 10/15 AML cases expressed CALLA with IE compared to 2/15 with IF. The corresponding figures for B1 were 5/15 and 0/15, respectively. Accordingly, normal myeloid precursor cells were CALLA-positive with IE but negative with IF. The discrepancy probably reflects the fact that, whereas both intracytoplasmatic and membrane-bound antigens are exposed in IE, only the latter are in IF. If the alteration of antigenic accessibility after fixation is considered, IE can safely be used for immunophenotyping of acute leukemia.