A murine monoclonal anti-idiotype antibody (anti-Id). Anti-Id 11D10 mimics a specific epitope of the high molecular weight human milk fat globule (HMFG) glycoprotein primarily expressed by human breast and some other tumor cells at high density. This specific HMFG epitope is identified by mAb BrE1, which was used as the immunizing antibody, or Ab1 to generate the anti-Id (Ab2) 11D10. Anti-ID 11D10 reacts specifically with breast tumor cells and with minimal reactivity with normal tissues. Vaccination with anti-Id 11D10 induces anti-anti-idiotype antibodies (Ab3) that may react with breast cancer cell lines expressing the HMFG membrane epitope.