The dose-response curves of benzidine derivatives calculated from Ames test data were analyzed by combining the least-squares method with cluster analysis. The calculations of the electronic structures of various benzidine derivatives have at the same time been performed by using the CNDO/2 method. The cluster analysis was carried out on various parameters concerning the electronic structure, such as total electron density, frontier electron density, the energy level of HOMO (highest occupied molecular orbital), and so on. It was found that the total electron density on the nitrogen atom gives a similar pattern of classification to that for mutagenic compounds in cluster analysis. In other words, it may be inferred that the nitrogen atoms of the benzidine derivatives play an important role in the metabolic activation of the compounds. Consequently, cluster analysis with a parameter of electronic structure should be a useful initial screening procedure for various analogous mutagenic compounds as well as carcinogenic compounds.