Introduct ionHartshorne and coworkers [l] have found recently that troponin, a protein which in the form of a complex with tropomyosin sensitizes actomyosin to changes in concentration of calcium ions (for review see [2]) can be separated at pH 1 .O into two fractions. One of them, troponin B, inhibited the Mg*'-stimulated ATPase activity of DAM* [3] independently of the concentration of Ca2+ and of the presence of tropomyosin whereas a second fraction, troponin A, had no influence on DAM per se, but sensitized troponin B to Ca 2+ ions in the presence of tropomyosin. Two similar fractions were later obtained by Schaub and Perry [4] with the use of SE-Sephadex chromatographyOur previous experiments also showed that preparations of troponin are heterogenous; moreover, disc electrophoresis and chromatography on DEAESephadex-A-50 column indicated the presence of more than two fractions [5]. By using the latter method the protein fractions were obtained and some of their properties are described in this paper.