Transcription from the as-1 element of the cauliflower mosaic virus is induced by salicylic acid (SA), an endogenous signal involved in plant defence responses. Electrophoretic mobility shift assays demonstrated that the binding of a tobacco cellular factor, named SARP, correlates with the SA-induced activation of transcription. SARP was shown to contain proteins immunologically related to TGA1a, a transcription factor previously cloned for its ability to bind to the as-1 element. The molecular mass of SARP was estimated to be 40 kDa by South-Western experiments. Treatment of the extracts with dissociating agents led to an increase in the DNA binding activity, suggesting the presence of an inhibitor that sequesters SARP. The DNA binding activity appeared sensitive to phosphatase treatment, suggesting a role for phosphorylation in the SA-induced gene activation. These results represent an analysis of immediate early response to SA and potentially elucidate the events of the SA signal transduction pathway.