The immobilization of microorganisms on ultrafiltration membranes made from polyacrylonitrile modified chemically with hydrazine hydrate and glutaraldehyde and their application to the biodegradation of phenol and cyanide contained in industrial wastewaters was examined. Three types of microorganism isolated from a mixed population of activated sludge adapted to phenol and cyanide decomposition were immobilized. Simulated phenol and phenol-cyanide wastewaters with a concentration of xenobiotics equal to their concentration in coke sewage were subject to ultrafiltration. The membranes were operated at a pressure ranging from 0.5 x 10(5) to 2.5 x 10(5) Pa, at a constant temperature of 298 K and a constant stirring rate in the bioreactor of 250 rpm. The results obtained were characteristic of each immobilized strain. The membrane with a mixture of microorganisms from the strains Agrobacterium radiobacter, Staphylococcus seiuri and Pseudomonas diminuta immobilized on its surface appeared to be the most effective. The coefficients of phenol and cyanide biodegradation were 36 and 20.3%, respectively.