Mucin-4 (Muc4) is a large cell surface glycoprotein implicated in the protection and lubrication of epithelial structures. Previous studies suggest that aberrantly expressed Muc4 can influence the adhesiveness, proliferation, viability and invasiveness of cultured tumor cells, as well as the growth rate and metastatic efficiency of xenografted tumors. Although it has been suggested that one of the major mechanisms by which Muc4 potentiates tumor progression is via its engagement of the ErbB2/HER2 receptor tyrosine kinase, other mechanisms exist and remain to be delineated. Moreover, the requirement for endogenous Muc4 for tumor growth progression has not been previously explored in the context of gene ablation. To assess the contribution of endogenous Muc4 to mammary tumor growth properties, we first created a genetically engineered mouse line lacking functional Muc4 (Muc4), and then crossed these animals with the NDL (Neu DeLetion mutant) model of ErbB2-induced mammary tumorigenesis. We observed that Muc4 animals are fertile and develop normally, and adult mice exhibit no overt tissue abnormalities. In tumor studies, we observed that although some markers of tumor growth such as vascularity and cyclin D1 expression are suppressed, primary mammary tumors from Muc4/NDL female mice exhibit similar latencies and growth rates as Muc4/NDL animals. However, the presence of lung metastases is markedly suppressed in Muc4/NDL mice. Interestingly, histological analysis of lung lesions from Muc4/NDL mice revealed a reduced association of disseminated cells with platelets and white blood cells. Moreover, isolated cells derived from Muc4/NDL tumors interact with fewer blood cells when injected directly into the vasculature or diluted into blood from wild type mice. We further observed that blood cells more efficiently promote the viability of non-adherent Muc4/NDL cells than Muc4/NDL cells. Together, our observations suggest that Muc4 may facilitate metastasis by promoting the association of circulating tumor cells with blood cells to augment tumor cell survival in circulation.