Transgenic mice expressing c-myc and v-Ha-ras specifically in the mammary gland under the control of the mammary specific promoter MMTV develop unifocal mammary tumors with a half time of about 46 days, and these tumors express high levels of osteopontin mRNA and protein. In order to evaluate the requirement for osteopontin expression by these tumors, we have crossed transgenic mice expressing these two oncogenes with mice with a targeted disruption of the osteopontin gene. Littermates expressing both myc and ras, and with either wild-type or disrupted OPN alleles were evaluated for tumor incidence and growth rate. Both of these parameters were found to be unaffected by a lack of osteopontin in the whole animal. Ras and myc expression level, measured at the level of mRNA, was not different in tumors of the two genotypes. Macrophage accumulation, while extremely variable among different tumors, did not correlate with the OPN status of the animals. Expression of the related gene BSP was not detected in any of the tumors, and was similar in bones of wildtype and OPN -/- mice. Similarly, the vitronectin gene was expressed at very low levels in tumors of either genotype. These results indicate that despite its high level of expression, OPN is either not required for mammary primary tumor formation and growth in this system, or can be replaced by molecules other than BSP and vitronectin in mice that totally lack osteopontin.