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To investigate the effect of ozone on two clones of white clover (Trifolium repens L. cv. Regal), differing in their sensitivity to ozone (O3), plants were cultivated during the vegetation periods 1997 (59 rain days; AOT40 from 20 May–7 Oktober: 5630 ppb.hr) and 1998 (81 rain days, AOT40 from 5 May–22 September: 4180 ppb.hr) according to the experimental protocol (UN/ECE, 1997, 1998a) of the International Cooperative Programme on Effects of Air Pollution on Natural Vegetation and Crops (ICP-Vegetation). Ozone episode related foliar injury was most pronounced in 1997. Protein content (only determined in 1998) decreased significantly (p ≤ 0.001) in S-clone leaves only, after an O3 episode of less than two weeks, but no effects on biomass and chlorophyll content were found. This gives rise to the question, whether the biomass ratio between the clones or chlorophyll content are appropriate parameters to determine O3 induced effects during cool and rainy summer seasons and whether other measurements of plant reaction (e.g. protein content of leaves) should be done in addition for a better description of O3 injury. Beyond that, the influence of factors modifying plant reaction to O3 (Level II) should be further elucidated. [ABSTRACT FROM AUTHOR]