There is little evidence to suggest that T lymphocytes are involved in the leukemic process in chronic myeloid leukemia (CML). A case of CML in blast phase is described in which T-cell surface antigens were detected by immunofluorescence on the patient's blasts using monoclonal antibodies. In order to determine that the T-cell blasts were derived from the original CML clone, cells bearing the T3 antigen were isolated by fluorescence-activated cell sorting and chromosome analysis was performed. All metaphases examined had the Philadelphia chromosome, confirming their origin from CML.