The Mg2+- and Ca2+-ATPase activities of myosin [ATP phosphohydrolase; EC 3. 6. 1. 3] were activated 10–15 fold, but the EDTA-ATPase activity was inhibited 80% by carboxamidomethylation of a specific cysteine residue of myosin by IAA****. The pH-activity curve of carboxamidomethyl myosin ATPase was very similar to that of untreated myosin ATPase. When myosin was treated with IAA in the presence of urea, 2 moles of cysteine residues in the Ile-CySH. Arg sequences per mole of myosin were modified. However, treatment of native myosin with IAA resulted in carboxamidomethylation of 1 mole of cysteine residue of this sequence per mole of myosin, and this modified the ATPase activity, as described above.