Human mucin MUC1 is abundantly expressed in some cancers of epithelia] origin and is largely restricted to the apical surface of secretory cells in normal tissues. It is, therefore, a potential target for cancer immunotherapy. In preparation for clinical trials, vaccines containing synthetic MUC1 peptides of different lengths and sequences mixed with various adjuvants or covalently attached, using different linker methods, to protein carrier keyhole limpet hemocyanin (KLH) were studied in mice. MUC1 peptides (containing 30 amino acids), plus adjuvants QS-21 or Bacillus Calmette-Guerin, were incapable of inducing antibody. However, MUC1 peptides conjugated to KLH (MUCl-KLH), plus QS-21, induced high titer antibody against the immunizing peptides and against MUC1-expressing tumor cells. Although T-cell responses, including delayed-type hypersensitivity, lymphocyte proliferation, and CTL, were not observed in mice immunized with these vaccines, significant protection from MUC1-expressing tumor cell challenge in mice immunized with MUC1-KLH was observed. Based on these studies, a vaccine containing MUC1-KLH conjugate prepared with m-maleimidobenzoyl-N-hydroxysuccinimide ester linker, plus QS-21, has been constructed for testing in clinical trials.