The GCN5 and ADA2 proteins are required for the activation function of a number of transcriptional activators in the yeast Saccharomyces cerevisiae. By using appropriate LexA fusion proteins we demonstrated that both proteins are required for part of the function of the GCN4, GAL4 and the VP16 transcriptional activation domains. Analysis of a gcn5 ada2 double disruption mutant did not reveal any additive effects, suggesting that the two proteins act in the same pathway. The GCN5 and ADA2 proteins can each activate transcription when directed to the promoter region of a reporter gene, but only in the presence of a wild-type ADA2 or GCN5 gene, respectively. The activation capacity is enhanced when the corresponding endogenous gene copy is disrupted. Taken together, these genetic data suggest that the two proteins interact and define one complex that mediates transcriptional activation. The function of this complex requires the bromodomain region of the GCN5 protein.