New Zealand White rabbits were infused with [3H]tyrosine for periods of 5--6 h and then different methods of extraction were applied for the purification of the main muscle proteins and protein fractions. Myosin (I), prepared from salt extraction of muscle mince, consistently had a higher specific radioactivity than did myosin (II), isolated by dissociation of actomyosin. Actins (IA) and (IB), extracted from acetone-dried powders prepared by different treatments of myosin-extracted muscle mince, gave specific radioactivities approx. 0.6 that of myosin (I) and 0.7 that of myosin(II). Actin (II), isolated by dissociation of actomyosin, had a specific radioactivity similar to that of myosin (II) from the same source, but higher than those of actins (IA) and (IB). The differences between the specific radioactivities of the proteins, in particular actin, purified by the various methods, are attributed to the loss of newly synthesized material of high specific radioactivity during the initial extraction procedures. It is suggested that actin (II) and myosin (II) are representative preparations for the total population of each protein and that, on this basis, myosin and actin have similar rates of synthesis. Total muscle protein, myofibrils, actomyosin and sarcoplasm were all found to have very similar specific radioactivities at the end of a 6 h infusion.