The action of trypsin, chymotrypsin and subtilisin on the adenosine-triphosphatase and actin-combining activities, as measured by viscometric means, of H-meromyosin were compared. 2. Subfragment 1 produced by prolonged tryptic digestion has a molecular weight of 129000. 3. The preparations isolated by gel filtration and actin combination were shown to be similar. 4. Subfragment-1 preparations possess appreciably higher adenosine-triphosphatase activities than H-meromyosin when related to total nitrogen. 5. Chromatographic and gelfiltration studies indicated that adenosine-triphosphatase activity is not distributed uniformly in all fractions of subfragment 1. 6. The Ca(2+)-activated adenosine triphosphatase of subfragment 1 was stimulated by thiol reagents in a similar fashion to myosin and H-meromyosin. 7. Subfragment 1 differed from myosin and H-meromyosin in that its adenosine triphosphatase was only slightly activated by Mg(2+) in the presence of actin. 8. A subfragment-1-like component was obtained by chymotryptic digestion of H-meromyosin. 9. The results obtained from enzymic and hydrodynamic studies and from amino acid analyses are compatible with the concept of one molecule of H-meromyosin giving rise to one molecule of subfragment 1 on proteolytic digestion.
