the authors investigated the effect of nitrosomethylurea (nmm) on the biodegradative power of active sludge in cleaning up solutions which contain caprolactam. the mutant forms of the biodegraders which appeared as a result of treatment with nmm adapted without difficulty in the natural biocenosis of the active sludge, since their seedability is essentially equal to the total microbial number of the samples selected. the oxidative operation of the aeration tank was judged from the chemical oxygen demand (cod) and the caprolactam content. use of the chemical mutagenesis method has made it possible to clean up a wastewater containing up to 600 mg/liter of caprolactam (that is, to increase the maximum concentration by a factor of two; to increase the oxidative power of the aeration tank two-fold; to raise the dehydrogenase activity two-fold compared with a control; and to improve the sedimentation properties of the sludge (sludge index, 40-50 cm**3/g as against 140 cm**3/g). the mutagen-treated sludge retains its properties for 5 months, which indicates the appearance of stable forms of the microorganisms