During 1988 and 1989, single and interactive effects of low concentrations of O3, SOS and NO2 (6–44, 9–63 and 10–43 μm−a as 24 h daily means) on two varieties of spring barley (Hordeum vulgare L., ‘Arena’ and ‘Alexis’) and spring wheat (Triticum aestivum L., ‘Turbo’ and ‘Star’) were investigated in factorial experiments using open-top chambers. Potted plants were exposed during the whole growing season to either charcoal-filtered air (CF; control) or CF supplemented with one (SO2, NO2) or two (O3) concentrations of the pollutants. While SO2 was applied continuously, fumigation with O3 and NO2 occurred every day sequentially from 8.00 to 16.00 and from 16.00 to 8.00 h, respectively. Analysis of variance indicated that from the main treatments O3 caused the most detrimental effects on growth and yield parameters. Depending on season and on variety, yield of wheat was reduced by O3 between 20% and 40% compared with the control treatment; a significant reduction of yield of barley (−15%) was only registered for ‘Arena’ (1988), while ‘Alexis’ appeared to be insensitive towards this gas. SO2 did not affect yield of barley significantly, while yield of wheat was either decreased (−7% for ‘Turbo’ in 1988) or increased (+11% for ‘Star’). NO2 did not alter yield of wheat significantly but it increased yield of barley (‘Arena’) by 15% in 1988. Significant interactive effects between the pollutants were observed only for a few parameters. Among these the most sensitive ones were the length of ears, the 1000-grain weight and the weight of straw. The interaction between the pollutants was mostly antagonistic, i.e. positive or negative effects of a single gas were cancelled or mitigated by the presence of other pollutants. However, a clear tendency towards a synergism was found for wheat in 1988: detrimental effects of O3 on yield (e.g, –26%;‘Turbo’) were aggravated by the addition of either SO2 (−38%) or, partially, NO3 (−31%) or both pollutants together (−44%). Before the plants were fully mature, visible symptoms were only observed on wheat after exposure to the highest O3 concentrations. The results showed that although effects of O3 dominated, these responses were strongly modified by the presence of one or both of the other gases. It is evident that yield losses of O3-sensitive crop species may be enhanced under certain circumstances by low concentrations of other pollutants. Therefore, estimations of yield losses from open-top chamber experiments should consider interactive effects between those pollutants which are relevant to ambient air.