CD7-positive acute myeloid leukaemias (CD7+AML) represent a distinct biological and clinical subtype of AML. The results of previous studies suggested that CD7 expression on myeloid leukaemic blasts might result from leukaemic transformation and maturation arrest of haemopoietic precursors at the stage of early myeloid differentiation when CD7 was transiently expressed. However, CD7+ myeloid progenitors have not yet been directly documented in normal human haemopoietic tissues. In this study, haemopoietic cells from 16 human fetal livers with a gestational age of 16-28 weeks were studied. Double myeloperoxidase (MPO) and CD7-positive cells could be demonstrated on 0-4% (mean 1.8%) of total fetal liver mononuclear cells (FLMC) by double cytochemical reaction of MPO and immunocytochemical staining of CD7. Simultaneous expression of CD7 and myeloid antigens (CD13 and/or CD33) could also be detected on 2.2-15.6% (mean 9.3%) of FLMC by dual-colour immunofluorescence flow cytometry analyses. CD13 and/or CD33 positive (CD13/33+) myeloid cells were positively selected by immunomagnetic bead separation system to a purity of 86.5-99.1% (mean 96.0%) of which < or = 3.3% were CD3+ cells and < or = 1.2% were CD19+ cells. Coexpression of CD7 was detected on 8.7-34.5% (mean 17.3%) of this CD 13/33+ cell population, but it was induced to decrease significantly after short-term in vitro culture with the differentiation-inducing agent phorbol ester (TPA). Coexpression of CD7 and CD13/33 could also be shown on a minor population of adult bone marrow and cord blood mononuclear cells (mean 3.9% and 1% respectively). In conclusion, the normal putative counterparts of blasts from CD7+ AML could be demonstrated in human haemopoietic tissues. The fact that CD7 expression tended to be lost after TPA stimulation suggested that CD7 was transiently expressed in early myeloid differentiation.