The kinetic mechanism for calf thymus histone acetyltransferase A has been determined from the initial velocity studies. The kinetic patterns at low substrate concentrations suggest that the reaction proceeds via two half-reactions as in a ping-pong pathway with the formation of an acetyl-enzyme intermediate. Such acetyl-enzyme has been isolated and found to be chemically competent. In addition, product inhibition patterns by coenzyme A are consistent with a hybrid ping-pong mechanism. These findings indicate that the acetyltransferase A from calf thymus has two separate and independent binding sites, one for each of the two substrates. Consequently, the mechanism constructed for the acetyltransferase A catalyzed reaction may be described as a double-displacement, two-site ping-pong mechanism.