The nuclear transcription factor AP-2 appears to be a key regulator mediating programmed gene expression during embryonic morphogenesis and adult cell differentiation. AP-2 has also been considered to be involved in epidermal gene regulation, but its precise role is not yet defined. The level of AP-2 transcripts increases during culturing of HaCaT keratinocytes preceding the expression of the differentiation-related gene keratin 4 (K4). The current study was aimed at investigating whether AP-2 transactivates K4 transcription. We cloned and sequenced the promoter region of K4 and found, in addition to canonical sequences, an AP-2 consensus site in the vicinity of the transcriptional start. In order to provide functional evidence for a regulation of K4 transcription by AP-2, we cloned various parts, which did or did not contain the AP-2 site of the K4 upstream sequence, into Cat reporter plasmids. These constructs were ballistically transfected into differentiating HaCaT keratinocytes. The determination of the resulting Cat activity revealed that the AP-2 site in the vicinity of the transcriptional start was functional for K4 transcription. Thus, the role of AP-2 in the process of keratinocyte differentiation appears to be considerable. In addition, further regulatory elements were found to be necessary for full transcription of K4.