The zinc finger gene MZF-1 is preferentially expressed in primitive hematopoietic cells and plays an important role in regulating myelopoiesis. Regulators of development are potential targets for neoplastic transformation. This study investigated whether unregulated expression of MZF-1 could function as an oncogene. Retroviral transduction and subsequent overexpression of MZF-1 resulted in loss of contact inhibition, loss of substrate dependence, and more rapid cell cycling in NIH 3T3 cells. The MZF-1-transformed 3T3 cells formed aggressive tumors in athymic mice. Disruption of the tight lineage- and stage-specific regulation of MZF-1 can result in neoplastic transformation of embryonic fibroblasts. Therefore, MZF-1 represents a novel oncogene.