The ubiquitous eps15 protein was initially described as a substrate of the EGF receptor kinase. Its functions are not yet delineated and this work provides evidence for its possible role in endocytosis. A novel anti-eps15 antibody, 6G4, coimmunoprecipitated proteins of molecular mass 102 kD. In human cells, these proteins were identified as the α- and β-adaptins of the AP-2 complex on the basis of their NH 2 -terminal sequence and their immunoreactivity with anti-α- and anti-β-adaptin antibodies but not with anti-γ-adaptin antibody. In addition, the anti-eps15 antibody coimmunoprecipitated metabolically labeled polypeptides with molecular mass of 50 and 17 kD, comparable to those of the two other components of the AP-2 complex, μ2 and σ2. Constitutive association of eps15 with AP-2 was confirmed by two sets of experiments. First, eps15 was detected in immunoprecipitates of anti-α- and anti-β-adaptin antibodies. Second, α- and β- but not γ-adaptins were precipitated by a glutathione-S-transferase eps15 fusion protein. The association of eps15 with AP-2 was ubiquitous and conserved between species, since it was observed in human lymphocytes and epithelial cells and in murine NIH3T3 fibroblasts. Our results are in keeping with a recent study showing homology between the NH 2 -terminal domains of eps15 and the product of the gene END3, involved in clathrin-mediated endocytosis of the pheromone α factor in Saccharomyces cerevisiae, and suggest a possible role for eps15 in clathrin-mediated endocytosis in mammals.