An indirect immunofluorescence technique with single and double labelling has been used to examine cultured human megakaryocytes for ABH antigens. This technique demonstrated the presence of these antigens on megakaryocytes and a population of small mononuclear cells that probably represent the differentiated precursors of megakaryocytes. In contrast to the intense homogeneous labelling with human anti-PlA1, the labelling with human anti-A, mouse monoclonal anti-A and anti-type 2H is light and heterogeneous, with many cells staining weakly or not at all. This variability in blood group ABH surface antigen expression appears to occur at the level of cell proliferation within colonies. While cells within an individual colony are homogeneous in fluorescence intensity, there is considerable variation between colonies. The progeny of individual megakaryocytes also appear uniform in ABH antigen expression.