We have previously raised two monoclonal antibodies (7B1, 14F11) recognizing the high-affinity leukotriene B₄ receptor, BLT1. They were presently used to determine receptor surface expression in the hematopoietic system. In peripheral blood, BLT1 was primarily recognized in granulocytes, monocytes and, to a lower extent, in certain lymphocytes except the CD4 subpopulation. The expression pattern was similar in bone marrow cells. In vitro differentiation of CD34⁺ progenitor cells induced BLT1 expression within 7 days, which remained constant up to day 17 when a further increase was measured and maintained up to day 20. BLT1 expression was modified by inflammatory mediators: LPS, TNFα, fMLP, as well as LTB₄ itself, caused a slight down-regulation at 30 min, an effect that was particularly marked with PMA, whereas the effect was least pronounced with IL-8. The antibodies have proved to be useful in an extensive mapping of BLT1 in both peripheral blood and bone marrow and as a tool to elucidate changes in the receptor expression.