The CBP protein acts as a transcriptional adaptor for many different transcription factors by directly contacting DNA-bound activators. One mechanism by which CBP is thought to stimulate transcription is by recruiting the histone acetyltransferase (HAT) P/CAF to the promoter. Here we show that CBP has intrinsic HAT activity. The HAT domain of CBP is adjacent to the binding site for the transcriptional activator E1A. Although E1A displaces P/CAF from CBP, it does not disrupt the CBP-associated HAT activity. Thus E1A carries HAT activity when complexed with CBP. Targeting CBP-associated HAT activity to specific promoters may therefore be a mechanism by which E1A acts as a transcriptional activator.