A new PR (pathogenesis-related) protein was isolated from tobacco leaves (Nicotiana tabacum cv. Samsun NN), reacting hypersensitively to tobacco mosaic virus (TMV), by zinc chelate chromatography and was therefore named Pz. Its reactivity toward several lectins indicated the presence of bound sugar residues. From the amino acid sequence of tryptic peptides, Oligonucleotide primers were derived which allowed the synthesis of Pz cDNA by PCR. Using this cDNA as probe, near full-length clones were isolated from a library made from poly(A)+ RNA purified from TMV-infected leaves. Sequence analysis revealed similarities with chitinases/lysozymes of various origins and the purified protein was, indeed, shown to hydrolyse different N-acetylglucosamine-containing substrates. Comparison of peptide and cDNA sequences indicated that Pz protein is synthesized as a pre-pro-protein, a seven-amino acid C-terminal peptide probably being involved in the vacuolar targeting of the protein. Pz mRNA and protein were demonstrated to accumulate strongly in TMV-infected tobacco leaves. Pz transcripts were also found in various tissues of healthy plants, indicating that Pz gene expression is developmentally regulated.