In MDCK cells, Golgi to basolateral transport of several membrane proteins has been found to involve a cytoplasmic domain determinant. In some cases (Fc receptor, lysosomal glycoprotein Igp120), the determinant appears similar to that required for endocytosis via clathrin-coated pits; for Igp120, elimination of a single cytoplasmic domain tyrosine both blocks internalization and results in apical transport. In other cases (LDL receptor), the determinant does not involve the cytoplasmic domain tyrosine required for endocytosis. Thus, contrary to current models, basolateral transport in MCDK cells occurs not by default but depends on one or more cytoplasmic domain determinants, the precise nature of which is unknown. For some proteins, it is closely related to coated pit determinants. The fact that many membrane proteins can reach the apical surface in the absence of this determinant suggests that signals for apical transport are widely distributed.