Surface and cytoplasmic expression of CD45 antigen isoforms in normal and malignant myeloid cell differentiation. The CD45 family contains protein tyrosine phosphatase (PTPase) activity and is expressed in one or more of its isoforms on all lymphohematopoietic cells. Considerable work has focused on CD45 expression by lymphoid cells, but minimal work has involved granulocytes. Granulocytic, or myeloid, cell differentiation is accompanied by a number of morphologic and immunophenotypic changes. This study used flow cytometric and immunocytochemical methods in conjunction with morphologic assessment to investigate the expression of CD45 isoforms during differentiation of normal and malignant granulocytic cells. On normal bone marrow cells, the quantity of surface CD45 did not change during earlier stages but did increase significantly at the terminal stages (bands and polymorphonuclear leukocytes [PMNs]). CD45RO (the low relative molecular mass [Mr] isoform) was very dimly expressed on immature cells but became increasingly brighter beginning at approximately the myelocyte stage. The high Mr isoform (CD45RA) was virtually absent from the cell surface at all stages. Only a small percentage (3-15%) of PMNs expressed surface CD45RA. However, there was a cytoplasmic pool of each isoform associated with membrane-bound granules found throughout differentiation, with remarkable increases in expression at the terminal stages. In the case of acute myeloid leukemias (AMLs), most cases expressed surface CD45RA with, or without, CD45RO, regardless of their French-American-British (FAB) classification. This appeared to be a stable process at diagnosis and relapse in individual patients and may therefore serve as a diagnostic aid. The biologic significance of this aberrant expression of CD45RA by malignant cells is unknown but raises important questions regarding the cellular processes of phosphorylation/dephosphorylation in normal and malignant cells. 