Chronic myeloid leukaemia (CML) is a clonal disorder of the pluripotent haemopoietic stem cell, the hallmark of which is the constitutively activated Bcr - Abl protein tyrosine kinase. During the initial chronic phase of CML the primitive multipotent leukaemic progenitor cells remain growth factor dependant and are capable of producing terminally differentiated cells. Although the available evidence suggests that Bcr - Abl directly affects signalling pathways involved in controlling the development of primitive haemopoietic progenitors the identification of the specific biological consequences of Bcr - Abl activity in these progenitors has been hampered by the lack of suitable systems modelling CML. By transfecting the multipotent haemopoietic cell line FDCP-Mix with a temperature sensitive mutant of Bcr - Abl we have developed the first working model that mirrors the chronic phase of CML. FDCP-Mix cells expressing Bcr-Abl tyrosine kinase activity remain growth, factor dependent and retain their ability to differentiate. Normal neutrophilic cells are formed in response to GCSF and GM-CSF. In addition, the transfected FDCP-Mix cells grown at the permissive temperature for Bcr - Abl tyrosine kinase activity display enhanced survival and proliferation in low concentrations of growth factor. These findings are consistent with the initial subtle changes seen in CML progenitor cells during the chronic phase and confirm that Bcr - Abl effects are context specific, i.e. they depend on the origin and developmental potential of the transfected cells. This questions the significance of studies in non-haemopoietic and differentiation blocked haemopoietic cells.