The value of flow cytometric detection of the intracellular lymphoid differentiation antigens CD3 and CD22 in the differential diagnosis of acute leukemia was assessed in cases of acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), and leukemic cell lines. Cells were fixed in 0.25% paraformaldehyde at 4 degrees C for 60 min, permeabilized with 0.2% Tween 20 at 37 degrees C for 15 min, then stained with CD3 or CD22 monoclonal antibodies by indirect immunofluorescence. Cytoplasmic CD22 was detected on greater than 20% (mean 55%; range 20-87%) of blasts from all 20 cases of precursor-B ALL analyzed. The percentage of cells with cytoplasmic CD22 was greater than that with membrane CD22 in all except 2 cases of precursor-B ALL. Cytoplasmic CD22 was not detected in 8 cases of precursor-T ALL, 4 T-leukemia cell lines, or in 7 cases of AML. In contrast, cytoplasmic CD3 was detectable by flow cytometry in all 8 cases of precursor-T ALL, but not in precursor-B ALL, pre-B leukemia cell lines, or in AML. These results confirm that cytoplasmic CD3 and CD22 are excellent markers of the early T and B lineages in ALL and can be reliably detected by flow cytometry. This technique should be a valuable addition to routine immunophenotyping for classification of acute leukemia.