Keratin filaments in simple epithelial cells are heteropolymers of keratin 8 (K8) and keratin 18 (K18), which can be stained by the monoclonal antibody (MAb) LE61. This antibody has been widely used to study keratin expression in normal and neoplastic tissues. In this study we have found that MAb LE61 does not react with individual keratin polypeptides either derived from natural sources or expressed as recombinant proteins in Escherichia coli. However, when K8 or K18 bound to nitrocellulose were incubated with complementary keratin they became reactive with this antibody. A mixture of K8 and K18 in solution also reacted strongly with the MAb LE61 in ELISA. These observations suggest that the antibody recognizes a discontinuous epitope on the keratin complex. The antibody also reacted with complexes of K8 and K18 with other keratins. To locate the epitope of this antibody we have expressed K8 and K18 fragments, deleted from the amino- and carboxyl-termini, as fusion proteins with glutathione S-transferase. These fragments were able to form a heterotypic complex with the complementary keratin. Binding of the MAb LE61 to these complexes mapped the two halves of the epitope on K8, between residues 353 and 367, and on K18, between residues 357 and 385. The two halves of the epitope appear to be in close association in the heterotypic complex since deletions from the amino-terminus did not influence the antibody binding. The highly conserved nature of this epitope in both type I and type II keratins could explain the MAb LE61 reactivity with complexes of K8 or K18 with other keratins.