A chimeric recombinant, containing the 270 bp left-terminal fragment of Adenovirus-2 (Ad2) inserted upstream from the -34 to +33 Ad2 major late promoter (Ad2MLP) element, has been used to characterize the transcription stimulatory element which is located at least 231 bp upstream from the E1A capsite in the left-end of Ad2 (Ref. 1). We demonstrate that this element, which acts in cis, possesses several properties characteristic of transcriptional enhancers. Firstly, it potentiates initiation of transcription from the capsite of the heterologous Ad2MLP and from "cryptic" sites often preceded by TATA box-like sequences. Secondly, although there is no critical distance requirement between the enhancer element and the Ad2MLP, the extent of stimulation decreases as the distance between the two element increases. However, in contrast to the other known viral or cellular enhancers which are bidirectional, the Ad2 enhancer is unidirectional, i.e. it potentiates the Ad2MLP element only when it is inserted in its "natural" orientation with respect to the direction of transcription. Using two convergent series of deletions, we have localized the Ad2 enhancer element within a 24 bp segment located at approximately 160 bp from the Ad2 left-end, i.e. 340 bp upstream from the E1A capsite. This 24 bp segment contains a sequence which exhibits a striking homology with the consensus sequence of several viral and cellular enhancers.