An IgM class monoclonal antibody Bsp-1 that selectively reacts with human basophils was used to label basophils in normal and leukaemic haemopoietic samples. The immunophenotype of Bsp-1+ basophils was determined using a panel of 21 IgG subclass monoclonal antibodies in two-colour immunofluorescence assays. Basophils expressed the leucocyte common antigen, HLA-ABC antigens and antigens defined by CD11 and CD13 monoclonal antibodies. Other myeloid cell (granulocyte-monocyte) associated anti-antigens, lymphoid cell surface determinants and HLA-DR antigens were not detected. Basophil preparations of 95-98% purity were obtained from the peripheral blood of patients with CML and umbilical cords using fluorescence activated cell sorting techniques. Purified basophils exhibited metachromatic staining with toluidine blue, alcain blue and astra blue. PAS staining was observed in 7% of cord Bsp-1+ cells and of 55% CML Bsp-1+ cells. Between 5% and 10% of basophils were chloroacetate esterase-positive which suggests that some Bsp-1+ cells are immature basophils.