AAMP (angio-associated migratory cell protein) shares a common epitope with alpha-actinin and a fast-twitch skeletal muscle fiber protein. An antigenic peptide, P189, derived from the sequence of AAMP was synthesized. Polyclonal antibodies generated to P189 readily react with AAMP (52 kDa) in brain and activated T lymphocyte lysates, alpha-actinin (100 kDa) in all tissues tested, and a 23-kDa protein in skeletal muscle lysates. The antibody's reactivity for alpha-actinin can be competed with the purified protein. Activation of T lymphocytes does not alter the degree of alpha-actinin reactivity with anti-P189 as it does for AAMP's reactivity in these lysates. Competition studies with peptide variants show that six amino acid residues, ESESES, constitute a common epitope in all three proteins in human tissues. The antigenic determinant is continuous in AAMP but discontinuous (or assembled) in alpha-actinin. alpha-Actinin does not contain this epitope in its linear sequence so reactivity is attributed to an epitope formed by its secondary structure. Limited digestion of the reactive proteins with thermolysin destroys anti-P189's reactivity for alpha-actinin while reactivity for recombinant AAMP is retained. Specificity of anti-P189 for human skeletal muscle fast fibers seen on immunoperoxidase staining may be explained by anti-P189's reactivity with a 23-kDa protein found only in skeletal muscle lysates. Its pattern of reactivity is the same as that obtained using monoclonal anti-skeletal muscle myosin heavy chain in type II (fast-twitch) fibers.