The effect of freezing on the stability of the estrogen receptors in human breast tumors was measured. Portions of three tumors kept frozen for 3 days after removal from the patients were analyzed by the sucrose density gradient method. The initial measurements demonstrated positive receptor values. The remaining portions were kept frozen for 30 days, and the receptor values were reevaluated. The 8S receptor was 80% destroyed under these freezing conditions. The 4S receptor was found to be more stable. The inactivation reflects a loss of specific binding since the protein concentration was similar in the analyses. These results strongly indicate that human breast tumor specimens should be analyzed as soon as possible or within 3 weeks after freezing.