The production of wheat cultivars with durable resistance to yellow rust is a major consideration in breeding programmes. Investigation of resistance components, the growth stages at which they operate, and mechanisms of resistance may yield information which would be useful in the rapid identification of durable resistance. Several components of resistance were investigated at various growth stages in a range of wheat cultivars exhibiting mature plant resistance. Spore gemination and penetration were reduced at adult growth stages on resistant compared to susceptible cultivars, but their usefulness as resistance components is questionable. Lengths of leaves colonised from initial point inoculations showed significant varietal differences at seedling stages. Measurements of colony sizes, pustule size and density, and latent period, generally correlated well with length of leaf colonised. Measurement of colonisation by a chitin assay detected varietal differences at seedling and adult growth stages. Seedling differences correlated well with other measurements of growth at that stage. The assay showed that, relative to a susceptible cultivar, some cultivars did not show increased resistance over the period of growth from the third (seedling) leaf to the flag leaf. In the same cultivars, the amount of host cell necrosis increased over the same period. The significance of this in terms of the relation between necrosis and resistance is discussed. An attempt to detect possible production or initial presence of antifungal substances in resistant cultivars using a spore germination assay was unsuccessful due to variability of germination levels. The possibility that lignification is involved in resistance to yellow rust was investigated, but results were inconclusive, and more sensitive methods of assaying lignin may be required.