There are two alpha-adaptin genes, alpha A and alpha C, which in brain encode proteins of of M(r) 108 x 10(3) and 104 x 10(3), respectively. Although both mRNAs can be detected on northern blots of brain and liver, the higher molecular mass polypeptide can only be detected on western blots of brain. Here we explain these observations by showing that alpha A is alternatively spliced and that the protein product in most tissues is different from the one expressed in brain in that it is missing 21 amino acids within the hinge region, giving it a similar mobility to that of alpha C. Monospecific antibodies were raised against the various alpha-adaptin isoforms and used to compare their distribution in cells and tissues. Both alpha A and alpha c are co-assembled into the same coated pits, and the larger isoform of alpha A is co-assembled with the smaller isoforms of alpha-adaptin, both in cells that naturally express it an in transfected cells.