Ovarian cyst fluid has been a valuable source of the mucins (traditionally termed "blood group substances") that were used for the elucidation of the structures of the ABO Lewis blood group determinants, but the identity of the mucin peptide core(s) carrying these carbohydrate specificities is not known. An ovarian cyst fluid mucin was purified, deglycosylated with HF and digested with trypsin or chymotrypsin to yield a number of peptides. Amino acid sequencing of these peptides yielded five different sequences which showed complete or partial homology to the MUC-6 apomucin deduced from DNA sequencing. As no other sequences were identified, it is concluded that MUC-6 is the major mucin core structure of ovarian cyst fluid mucin.