Abstract. Originally identified as a basolateral domainspecific integral plasma membrane protein of the rat hepatocyte, CE9 mRNA and protein were also detected at high levels in the testis of the rat by Northern and Western blotting and immunoprecipitation . CE9 proved to be a domain-specific integral plasma membrane protein of the rat spermatozoon : on testicular spermatozoa, it was concentrated within the posterior tail domain of the plasma membrane, whereas on vas deferens spermatozoa, CE9 was concentrated within the anterior tail domain . This change in the localization of CE9