Endo-1,4-beta-d-xylanases (EC 3.2.1.8, xylanases) and xylanase inhibitors, that is, TAXI (Triticum aestivum xylanase inhibitor), XIP (xylanase inhibiting protein), and TLXI (thaumatin-like xylanase inhibitor) type xylanase inhibitors, which naturally occur in cereals, are believed to be at the basis of a significant part of the variability in biotechnological functional properties of cereals. Xylanase inhibitors in particular affect grain functionality during processing and in animal feeds when xylanases are used to improve processing parameters and product quality. In the present study the variability of xylanase, TAXI, and XIP activities was quantified in different cereals, including different wheat types [common wheat (Triticum aestivum L.), durum wheat (Triticum durum Desf.), spelt wheat (Triticum spelta L.), einkorn wheat (Triticum monococcum L.), and emmer wheat (Triticum dicoccum Schübler)], barley (Hordeum vulgare L.), rye (Secale cereale L.), and oat (Avena sativa L.), and the contribution of genotype and environment to this variability in common wheat was estimated. Substantial differences in xylanase, TAXI, and XIP activities exist between the different cereal types and varieties. Under the experimental conditions of this study, the durum wheat samples show very high xylanase activities compared to the other cereals. High TAXI and XIP activities were measured in, for example, common wheat, spelt wheat, and rye, whereas low activities occur in barley and oat. For wheat, a significant part of the variability in inhibitor levels can be explained by genotype, whereas xylanase activity is most strongly determined by environment. The results obtained suggest that plant breeders and industry to certain extent can select for wheat varieties with high or low xylanase inhibition activities, but the relatively high contribution of the genotype-environment interaction term to the total variability in inhibition activities indicates that TAXI and XIP activities are not very stable breeding parameters.