We have studied the organization of clathrin during its transport in axons. Using immunoprecipitation techniques we have confirmed earlier findings that clathrin is transported as part of slow component b, but we also detect small amounts of clathrin in fast component. As fast component is known to correspond to the transport of membraneous material, including coated vesicle membrane components, our findings suggest that some clathrin in axons undergoes transport in the form of coated membranes and that a portion of the clathrin delivered to axons and axon terminals arrives by way of fast component. The organizational form of clathrin in slow component b (SCb) was examined in more detail, as it is thought to represent a non-membrane-associated species, is relatively long-lived, and at any instant represents the major transport species in axons. We used nondenaturing immunoprecipitation methods with stringent wash procedures to identify other SCb proteins that interact with clathrin. The immunoprecipitates contained major labeled bands that corresponded to clathrin heavy and light chains, along with a prominent 70-kDa band and several minor bands that ranged in apparent Mr from 70,000 to 150,000; the 70-kDa band was shown to be the ATP-dependent uncoating protein by two-dimensional gel electrophoresis. A very similar profile of polypeptides was also immunoprecipitated from extracts of cultured neurons. The results from a variety of control immunoprecipitations, including the use of antisera preadsorbed with purified clathrin trimers or clathrin light chains, indicate that coprecipitation of clathrin and uncoating protein with the other 70,000-150,000-Da polypeptides from SCb reflects specific interactions. Including exogenous uncoating protein in the lysis buffer had no detectable effect on the levels of endogenous uncoating protein recovered in the immunoprecipitates, indicating that complexes of clathrin, uncoating protein, and the other coimmunoprecipitating SCb protein existed in the intact neurons prior to lysis. Finally, a specific and functional association is further supported by the release of uncoating protein, but not the other 70,000-150,000-Da polypeptides, from the immunoprecipitated complexes on the addition of ATP. Collectively, these observations provide the first direct evidence of interaction between clathrin and uncoating protein in intact cells, lend strong support to the concept that uncoating protein plays an intimate role in clathrin dynamics within cells, and reveal a family of 70,000-150,000-Da polypeptides that form a stable nonmembranous association with clathrin in intact cells.