A distinctive, thick walled, postanal sac is found midventrally on the caudal peduncle of most gray whales. The structure appears to be unique to this cetacean and is a relatively unknown organ, with no known function. This study describes partial physicochemical characterization of the fluid contents in the sac. The major chemical component is a unique glycoprotein of molecular weight greater than 150,000 daltons, as determined by gel filtration, analytical ultracentrifugation, acrylamide gel electrophoresis, and colorimetric analyses. A second component of lesser molecular weight, appears to be predominantly protein.