Multiparity has been linked with protection against breast cancer. T cells from biparous women, but not T cells from nulliparous women or men, specifically proliferated in response to core peptide sequences of a human breast cancer-associated mucin (MUC-1). Two of the nulliparous women were retested during the first trimester of their first pregnancy, and their T cells proliferated specifically in response to MUC-1 mucin. These observations support the hypothesis that there is a natural immunization against MUC-1 peptide epitopes during pregnancy which provides some protection against the development of breast cancer. These data also suggest that certain MUC-1 synthetic peptides might be effective components of "vaccines" for therapy or prevention of breast cancer.