Initial velocity studies have been carried out on protein methylase II (S-adenosyl-L-methionine:protein O-methyltransferase, EC 2.1.1.24) purified from calf thymus, using bovine pancreatic ribonuclease as the protein substrate. Initial velocity patterns converging at a point on or near the extended abcissa were obtained with either ribonuclease or S-adenosyl-L-methionine as the variable substrate. Inhibition by the product S-adenosyl-L-homocysteine was linear competitive against both S-adenysyl-L-methionine and ribonuclease, the apparent inhibition constants being dependent on the concentration of the nonvaried substrate. Adenosine was an inhibitor of the reaction, the inhibition being linear competitive against both S-adenosyl-L-methionine (Ki/1.2 times 10-3 mol/1.) and ribonuclease (Ki/4.6 times 10-3 mol/1.). These results are consistent with a random mechanism for the protein methylase II reaction in which the rate-limiting step may be the interconversion of the ternary complexes and all other steps may be in equilibrium. The limiting Michaelis constants for S-adenosyl-L-methionine and ribonuclease are 0.87 times 10-6 and 2.86 times 10-4 mol/1., respectively. The dissociation constants of S-adenosyl-L-homocysteine for its reaction with the free enzyme was 1.03 times 10-6 mol/1. Thus it has about equal affinity for calf thymus protein methylase II as S-adenosyl-L-methionine.