The acidic transcriptional activation domain of the herpes virus activator VP16 requires an accessory protein complex for function, termed an adaptor. Although the activation domain of the yeast activator HAP4 is also highly negatively charged, its function is independent of at least one component of the adaptor complex, ADA2. In this study, we have used an in vitro inhibition assay to determine whether the activation domains of VP16 and HAP4 use a similar mechanism to potentiate transcription. Both domains had potent activation ability, indicating a similar strength of action. However, the capacity of each domain to inhibit activation of a heterologous test promoter (dA/dT) was sharply dissimilar. VP16 selectively inhibited activated transcription of dA/dT, without affecting basal transcription, implying that VP16 and the activator protein of the dA/dT promoter share a mechanism for activation. In contrast, HAP4 was totally unable to inhibit activated transcription of the dA/dT template. In the second part of the study, a genetic selection was used to obtain mutations in putative cofactor genes for HAP4. The spectrum of phenotypes caused by these mutations was strikingly different than mutations in the adaptor for the VP16 activation domain. These results strongly suggest that HAP4 and VP16 have distinct cofactor requirements, although they are both acidic activators.