A murine monoclonal IgM erythrocyte antibody appeared to have anti-P (anti-globoside) specificity. The antibody was a relatively weak cold agglutinin, but a strong haemolysin and its reactivity with red cells was markedly enhanced by enzyme treatment. This antibody was used to study the cell and tissue distribution of globoside. Globoside was not only detectable on red cells and erythroblasts, but also on endothelial cells and on subsets of platelets, megakaryocytes and fibroblasts. It was not detectable on granulocytes, monocytes and most peripheral blood lymphocytes. Neither was it present on erythroblast precursors (CFU-E, BFU-E), pro-erythroblasts or on the cells of the pro-erythroblastic cell lines K562 and HEL. However, K562 cells expressed globoside when induced to mature into erythroblasts by sodium butyrate. Cells of patients with various leukaemias were also tested. A significant number of positively reacting cells was frequently (six out of 18) seen in cases with a CML blast crisis (CML-BC) and rarely in AML (four out of 37 cases). In CML-BC the P-positive cells were probably erythroblasts and/or megakaryoblasts. Thus, globoside appeared to be an interesting marker in CML-BC of the erythroblastic or mixed erythroblastic-megakaryoblastic type.