Actobindin is an 88-amino acid polypeptide, containing two almost identical repeated domains of 33 and 34 residues. Depending on the molar ratios in which they are mixed, actobindin binds either one or two actin molecules. We cross-linked actobindin and actin in the 1:1 complex, using the zero-length cross-linker 1-ethyl-3(3-dimethylaminopropyl)carbodiimide. The cross-linked peptides were purified after consecutive CNBr cleavage and trypsin and Staphylococcus protease V8 digestions, and the cross-linked side chains were identified by amino acid sequencing. Isopeptide linkages were formed between residues Glu-100 of actin and Lys-16 of actobindin. In addition, we found a connection between one or more of the acidic residues 1,2, or 3 of actin and Lys-16 and Lys-52 of actobindin. The cross-linked regions in actobindin contain Leu-Lys-His-Ala-Glu-Thr motifs, similar to sequences observed in several other actin-binding proteins.