Differential display of mRNA has been improved by developing a two-step PCR amplification procedure. The modified differential display has been applied to identify early alterations of mRNA expression in response to auxin treatment of tobacco seedlings. This approach has led to the isolation of four fragments corresponding to auxin-up-regulated mRNAs. One, named GO15-13, shows significant homology with the 3' end of the coding region of the soybean SAUR X10A. The three other fragments present no homology with sequences available in the databases and constitute potential new early auxin markers.