MUCl epithelial mucins are produced by both normal and malignant epithelial cells. Serum proteins reactive with monoclonal antibodies against MUCl mucins were studied using several techniques. Separation of proteins by native PAGE showed that anti-MUC1 core protein antibodies reacted with a high Mr mucin but also with a 70-kD protein (p70) in normal women and women with ovarian cancer. After purification by gel filtration, p70 was not reactive in a double-determinant ELISA (CASA) and only weakly reactive in an inhibition assay. Serum CASA levels increased during pregnancy but p70 disappeared. Neuraminidase treatment of serum resulted in a greater increase in CASA in normal women and in ovarian cancer patients with low initial CASA than in ovarian cancer patients with high CASA and pregnant women (p < 0.05). These findings suggest that pregnancy and ovarian cancer-derived mucins are less heavily sialylated than mucin derived from normal tissues. An inhibition assay was developed which was more sensitive, but provided no diagnostic advantage over CASA. MUC1 is present in the serum of all women, reactivity in assays utilizing core protein antibodies is probably dependent not only on molar concentration but on the degree of exposure of peptide epitopes.