In a search for biochemical markers indicating the impact of ozone on plants. Pathogenesis related (PR) proteins were studied. The following observations were made: (1) PR-proteins are present in gymnosperms, exemplified by Norway spruce (Picea abies) and (2) ozone induces PR-proteins in Norway spruce. Studies were made by Western blot analysis using antibodies raised against seven PR-proteins; five from tobacco (PR-1a, -1b, -1c, -P and -Q) and two from maize (PRm 6 and 7). Each antibody recognized specific proteins in spruce needles. A more detailed study was done with the antibodies against tobacco PR-P (endochitinase) and maize PRm 6 (β-1,3-glucanase). Exposure to ozone increased the contents of proteins recognized by both antibodies. In control trees, anti-PR-P antibody recognized a protein of about 28 kD while in the ozone-exposed trees a form with a slightly faster migration rate in SDS-PAGE predominated. In contrast, in the range of 34–44 kD, a family of proteins was recognized by anti-PRm 6 antibody upon induction by ozone. In this same size range, a strong bund of 37 KD appeared in the total protein pattern, corresponding to about 5 to 10% of total protein. N-terminal sequencing revealed a typical homology pattern shared by β-1,3-glucanases. This was also reflected by the enzymatic level Where ozone was shown to increase β-1,3-glucanase activity. By inducing pathogenesis-related proteins, ozone may modify the response of spruce to pathogens.