We have deduced the 1675-amino acid sequence of rat clathrin heavy chain from cDNA clones and predict a protein of Mr 191,569. We have established the polarity of the heavy chain and assigned sequence positions to several structural landmarks of the clathrin leg. The terminal domain at the distal end of the clathrin leg is at the amino terminus of the heavy chain. It is connected to the distal segment by a flexible "link" from Tyr-479 to Arg-523. There is an unusual sequence at the carboxyl terminus that may form the globular projection at the vertex of the clathrin trimer. We suggest that a possible site of heavy-chain-light-chain interaction is located in the proximal segment. Comparison with other partially sequenced mammalian clathrin heavy chains shows that the primary structure is highly conserved. The heavy chain is unrelated to other classes of structural proteins.