A biotest to investigate wastewaters for the presence of nitrification-inhibiting substances has been developed. The principal feature of the test system is a packed-bed fixed-film biological reactor operated as a differential reactor. The test has been used to determine the effects on nitrification of wastewaters especially from textile and leather industries. Inhibition could be found even when the wastewater was diluted considerably. Tannery sewage may cause particularly severe problems in biological wastewater treatment, as the degree of inhibition of this wastewater has been observed to be similar to that of a solution of 2 mg/L allythiourea.