The tyrosine kinase inhibitor (= tyrphostin) AG 555 selectively interferes with viral transcription in bovine papillomavirus type 1 transformed fibroblasts. This was accompanied by cell cycle arrest and suppression of cyclin-dependent kinase activity. Concomitant with inhibition of viral transcription, c-jun was strongly up-regulated, which was consistent with the observation that AG555 treatment also led to an activation of the MAP kinase pathway by enhancing phosphorylation of JNK and p38. Increased JNK and p38 activity resulted in higher phosphorylation rates of the AP-1 family members c-Jun and ATF-2. Scanning the BPV-1 genome for potential binding sequences, an intragenic AP-1 site (“BAP-1”) within the E7 open reading frame was detected. Enhanced dimerization of phosphorylated ATF-2 together with c-Jun and binding to BAP-1 seems to be responsible for viral dysregulation, since both suppression of BPV-1 and c-jun mRNA induction could be almost entirely abrogated by simultaneous treating with SB 203580, an inhibitor of p38 MAP kinase activity. Moreover, dissecting the complex transcriptional pattern of episomal BPV-1 with specific primer sets for RT-PCR analysis, the repressive effect could be attributed to a selective down-regulation of the mRNA encoding the E2 transactivator function in favor to the E2 repressor, whose mRNA level remained constant during AG555 treatment. These data indicate that tyrphostin AG555 disturbs the balance of negative and positive regulatory factors necessary to maintain the homeostasis of a virus-transformed phenotype.