A temperature sensitive abl protein tyrosine kinase gene was transferred into a multipotent haemopoietic stem cell line, and the primary biological effects of expression of the gene were examined at the permissive and non-permissive temperatures. Unlike previous studies in factor-dependent cell lines, we found that expression of the functional abl protein tyrosine kinase did not lead to growth autonomy. Furthermore, the cells were still able to undergo terminal myeloid differentiation. However, expression of the functional gene did lead to a delay in maturation with a concomitant increase in cell production, had a modest effect in terms of delayed apoptosis particularly when the cells were maintained at a high cell density, and slightly increased the response to sub-optimal concentrations of IL-3. In many respects, therefore, the effects of abl protein tyrosine kinase in these cells mimics the effect of bcr/abl in primary haemopoietic cells where growth factor independence and an aberrant differentiation profile are relatively late events in clonal evolution and are not intermediate consequences of activation of the abl gene.