The Gcn5 histone acetyltransferase (HAT) is part of a large multimeric complex that is required for transcriptional activation in yeast. This complex can acetylate in vitro and in a Gcn5-dependent manner both nucleosomal and free core histones. For this reason it is believed that part of the function of the Gcn5.Ada complex is chromatin remodeling effected by histone acetylation. The roles of the other subunits of this complex are not yet known. We have generated mutated Gcn5 proteins with severely attenuated in vitro HAT activities. Despite their apparent loss in HAT activity, these GCN5 derivatives complemented all the defects of a gcn5 strain. We have shown that when these mutated proteins were produced in yeast cells in the absence of another component of the complex, Ada2, their activity was still compromised. By contrast, when produced in the wild type context, they were partially capable of acetylating free histones and were even more active when nucleosomal arrays were used as substrates. Kinetic enzymatic analyses showed that the rate of catalysis by Gcn5 was enhanced when the mutated proteins were produced in yeast in the presence of Ada2. Because Ada2 is required for the assembly of Gcn5, we conclude that one role for components of the Gcn5.Ada complex is the potentiation of its HAT activity.