A new mucin antigen was detected in a mucinous cystadenoma of human ovary. An antiserum produced in rabbits against the crude cyst fluid, following appropriate absorptions, showed immunoreactivity exclusively with tumour epithelium, particularly endocervical type epithelium. The antigen responsible for the immunoreactivity was isolated in the fraction with density 1.45g/ml following density gradient ultracentrifugation in cesium chloride, which indicated that it was indeed a glycoprotein. The native glycoprotein was very large since it was excluded from Sepharose CL-2B. The complex could be dissociated following reduction with dithiothreitol. The subunits were included in Sepharose CL-2B and the position of elution would indicate a molecular weight in the order of 1-5 x 10(6). This suggests the native antigen was a complex made up of subunits held together by disulphide bonds. Amino acid analysis of the new antigen showed a resemblance with intestinal mucins, as two-thirds of the protein consists of threonine, serine, proline, alanine and glycine. This similarity in peptide core may explain the potential of the epithelium in ovarian mucinous cystadenoma to produce inappropriate intestinal mucins during the process of malignant transformation.