Inhibition of tyrosine kinases is a possible approach for the treatment of cancer. We have investigated the catalytic mechanism of the epidermal growth factor receptor tyrosine kinase (EGF-RTK) in order to obtain information for use in structure-based searching for inhibitors. Initial rate studies imply that EGF-RTK forms a ternary complex together with ATP and peptide substrate. Investigation of pH and temperature dependence suggests that the kinase reaction requires the ionised form of a carboxylate (pK = 6.3) and the protonated form of another group (pK = 9.1). These characteristics are consistent with a mechanism where the carboxylate of Asp813(pK = 6.3) facilitates deprotonation of the tyrosyl hydroxyl of the peptide substrate, activating it as a nucleophile to attack the gamma-phosphorus of ATP which interacts with a protonated enzyme side-chain (pK = 9.1), possibly the guanidinium group of Arg817. This proposed catalytic mechanism was used to define a query when searching for inhibitors in a database of predicted three-dimensional structures. The procedure involved searching for compounds that mimic the ATP gamma-phosphate, tyrosyl hydroxyl and the tyrosyl aromatic ring, all of which seem to interact strongly with the enzyme during catalysis. This search allowed identification of inhibitors of EGF-RTK which were used to define queries for two-dimensional searching of a larger database, leading to the discovery of 4-(3-chloroanilino)quinazoline (CAQ) which is a potent inhibitor (Ki = 16 nM) of the enzyme. The compound is believed to be the first representative from a new structural class of anilinoquinazoline tyrosine kinase inhibitors. It follows competitive kinetics with respect to ATP and noncompetitive kinetics when the peptide is varied, implying that it functions as an analogue of ATP. CAQ is a novel and potent lead in the search for tyrosine kinase inhibitors as potential agents for the treatment of cancer.