The v-myb-containing viruses AMV and E26 induce the proliferation of myelomonocytic cells. The E26 Myb protein, by virtue of its fusion to Ets, is also able to transform multipotent haematopoietic cells (MEPs). We have examined the biological effects of substituting the v-Myb transactivation domain with the strong acidic activator domain from the C-terminus of the HSV-1 VP16 protein. In the absence of Ets, deletion of the transactivation domain destroyed the ability of v-Myb to stimulate transcription and to transform cells, whilst the substitution of the VP16 transactivation domain into v-Myb resulted in a greatly enhanced transactivation potential and altered TATA box binding protein (TBP) binding properties. In spite of these functional differences, the v-Myb VP16 protein regained the ability to transform myeloid cells with the same characteristics as wild type v-Myb. A construct encoding v-Myb VP16 fused to v-Ets was still capable of inducing leukaemia and of transforming both myeloid cells and MEPs in vitro, although the latter cells exhibited an altered phenotype. Our results demonstrate that the transformation of myeloid cells by v-Myb is largely independent of the type and potency of the transactivation domain it contains, whereas transformation of MEPs by the Myb-Ets fusion protein has more stringent transactivation requirements of Myb.