Human papillomaviruses (HPV) replicate in keratinocyte but not fibroblast cells. Several factors, including AP1 (Jun/Fos), contribute to the cell-type specific transcription of HPV genes. The binding of AP1 upstream of the HPV type 18 early gene E6 is essential for transcription of the early genes. Here we show that AP1 levels are low in early passage human fibroblast extracts. In contrast, human keratinocyte extracts contain high levels of AP1. In agreement with this, in vivo an AP1-dependent promoter is more active in keratinocytes than in fibroblasts. Pulse chase experiments indicated that Jun and Fos are relatively stable in human keratinocyte cells after serum induction, whereas in early passage human fibroblasts they are rapidly broken down. Nuclear extracts of these fibroblasts contain a cysteine proteinase which can degrade AP1. Furthermore, the activity of a cathepsin B-like cysteine proteinase is elevated in these human fibroblast extracts relative to other cell types. Interestingly, after several passages in culture the fibroblasts lose this proteinase activity and the amount of AP1 increases. Taken together, these results suggest that the quantitative difference in AP1 proteins between human keratinocytes and fibroblasts is due to a difference in protein stability. The cathepsin B-like cysteine proteinase is a candidate for a role in the unusually rapid breakdown of AP1 in early passage human fibroblast cells. Low levels of AP1 in the fibroblasts correlate with the low activity of AP1-dependent promoters, like that of HPV-18, in these cells.