The localization of specific antibodies against troponin components, i.e., troponin T (TN-T), troponin I (TN-I), and troponin C (TN-C), was studied by the use of an electron microscope. Every antibody was distributed along the thin filament with a period of 38 nm. Staining with anti-TN-I or anti-TN-C formed narrow striations. The location of the first striation was 26 nm from the free end of the thin filament. The width of individual striations formed by anti-TN-T was 14–20 nm. The riband-side end of each striation coincided with the location of anti-TN-I or anti-TN-C.