4-Chloroaniline transformation products, generated in soil and by partially purified enzymes of the soil fungus Geotrichum candidum were compared using autoradiochromatography. The results of this comparison indicated that these enzymes constitute a useful model system for investigation of aniline susceptibility to polymerizing transformations. Forty-three variously substituted anilines were exposed to the fungal enzymes and the reaction products were analyzed by spectrometric and gas chromatographic methods. Susceptibility to enzymic transformation increased with increasing electron density at the amino group. Electron-releasing substituents increased yields of higher polymers, but decreased the yields of azobenzenes. Anilines substituted by electron-attraction groups in both ortho positions resisted enzymic transformations. The results allow predictions about the environmental fate of pesticide-derived aniline residues.