The MUC1 tumor antigen is overexpressed on most breast tumors and metastases. It interacts with signaling proteins such as the ErbB kinases and beta-catenin, and is involved in mammary gland oncogenesis and tumor progression. Herein, we report a novel interaction between MUC1 and adenomatous polyposis coli (APC), a tumor suppressor involved in downregulating beta-catenin signaling. Initially identified in colorectal cancer, APC is also downregulated in breast tumors and presumably involved in mammary carcinogenesis. MUC1 and APC co-immunoprecipitate from the ZR-75-1 human breast carcinoma cell line and co-localize in mouse mammary glands and tumors. These studies also indicate that the association of MUC1 and APC may be increased by epidermal growth factor stimulation. Intriguingly, the co-immunoprecipitation of MUC1 and APC increases in human breast tumors and metastases as compared to adjacent normal tissues, indicating that this association may play a role in the formation and progression of breast tumors.