The polymorphic epithelial mucin (PEM) is found as a cell associated transmembrane protein with an extracellular domain made up largely of tandem repeats and also as a soluble form in some body fluids and culture supernatants. To determine whether the soluble form can arise without the mechanism of alternative splicing mouse cells have been transfected with an expression construct containing the full-length cDNA, and the supernatants of the transfectants analyzed for the presence of the mucin. The presence of mucin in the supernatants could indeed be detected in a radioimmunoassay and by immunoprecipitation using monoclonal antibodies to the tandem repeat region of the core protein, indicating that release of the soluble form can occur without alternative splicing. The soluble form was not however precipitated with a polyclonal antiserum to the cytoplasmic tail, suggesting that it was released from the membrane by the action of a protease.