A series of 4-substituted quinazolines and related compounds have been prepared and evaluated for their ability to inhibit the tyrosine kinase activity of the epidermal growth factor receptor on a phospholipase C-gamma 1-derived substrate. The results show a narrow structure-activity relationship (SAR) for the basic ring system, with quinazoline being the preferred chromophore and benzylamino and anilino the preferred side chains. In the 4-anilino series, substitution on the 3-position of the phenyl ring with small lipophilic electron-withdrawing groups provided analogues with enhanced potency. Two series of compounds [4-(phenylmethyl)amino and 4-(3-bromophenyl)amino] were studied to determine SARs for quinazoline substituents. In the more active 4-(3-bromophenyl)amino series, electron-donating groups (NH2, OMe) at the 6- or 7-position increased activity, in a pattern consistent with a requirement for high electron density in the vicinity of the 8-position of the quinazoline ring. The 6,7-dimethoxy derivatives were the most effective in both series, with the 4-(3-bromophenyl)amino derivative (3) having an IC50 of 0.029 nM, making it by far the most potent reported inhibitor of the tyrosine kinase activity of the epidermal growth factor receptor enzyme.