The breast epithelial mucin is one of the currently used serum breast cancer markers. Immunoassays detect elevations of this marker in breast cancer relapse; however, values obtained do not usually correlate with breast tumor load and false negatives are frequent. The causes for such results were investigated in an immunodeficient mouse grafted with transplantable human breast tumors. Increases in serum breast epithelial mucin values are only found after a significant threshold of tumor load is reached and only when a highly increased release of the marker is created by radioimmunotherapy. This indicates that substantial amounts of the marker must be released to overcome clearance. Further, a hepatic clearance mechanism is demonstrated since carbon tetrachloride toxicity increases breast epithelial mucin levels in tumor bearing mice.