Undifferentiated blasts from a Ph'-positive chronic myelogenous leukaemia (CML) in terminal metamorphosis were reacted in an indirect immunofluorescence test with antilymphocytic globulins (AHLGs), raised against cultured lymphoblasts, thoracic duct and peripheral blood lymphocytes from healthy donors. After proper myeloid and/or monocytic absorptions the AHLGs interacted strongly with the undifferentiated blasts of CML, while this was not true for parallel controls with non-lymphoid leukaemias, both acute and chronic. The intensity of fluorescence, as determined by the use of a microfluorimeter, on these agranular blasts was comparable to the positivity of lymphoid cells from acute and chonic lymphatic leukaemias. These findings lend further support to the conception of a lymphoblast-like variety of terminal blastic crisis in chronic myelogenous leukaemia.