Failure of Botrytis cinerea to penetrate the intact surface of primary wheat leaves was associated closely with the production of papillae and alterations in the upper epidermal walls (“haloes”) and adjacent lateral walls. Autoradiography, spectroscopy, fluorescence microscopy and histochemical tests indicated that lignification occurred at these sites. Papillae also contained a substantial amount of polysaccharide, callose and cellulose being indicated by histochemical tests. The apparent time of initiation of lignification was dependent on the detection method employed. Autoradiography of the surface of extracted epidermal strips taken from leaves fed with 3H-phenylalanine or 14C-cinnamic acid indicated that lignification of the upper epidermal wall occurred at an early stage, being detectable as soon as papillae were visible. Papillae, haloes and altered lateral walls were resistant to enzymes which degrade normal epidermal walls. This resistance, which was detectable as soon as papillae were formed, could be destroyed by chemical delignification but not by removal of wall-esterified phenolic acids. Papillae, haloes and lignification were also associated with attempted penetrations by Alternaria tenuis, Septoria apiicola, Botrytis allii and B. fabae. Drop-diffusate tests for phytoalexins using B. cinerea were negative. The possible ways in which lignification, papillae and haloes may be involved in resistance are discussed.