We have derived and mated separate strains of transgenic mice that carry either the v-Ha-ras or the c-myc gene driven by the mouse mammary tumor virus (MMTV) promoter/enhancer. Mice carrying the MMTV/v-Ha-ras transgene manifest two distinct disturbances of cell growth. The first, a benign hyperplasia of the Harderian lacrimal gland, is diffuse, involves the entire gland, and likely requires only the abnormal action of the v-Ha-ras gene. The second involves the focal development of malignancies of mammary, salivary, and lymphoid tissue and likely requires additional somatic events. When the MMTV/v-Ha-ras and MMTV/c-myc strains are crossed to yield hybrid mice, their joint action results in a dramatic and synergistic acceleration of tumor formation. Since these tumors arise stochastically and are apparently monoclonal in origin, additional somatic events appear necessary for their full malignant progression, even in the presence of activated v-Ha-ras and c-myc transgenes.