The growing awareness of environmental problems provided the stimulus for this 4th International Symposium on Biotechnology, Interbiotech '90, to address many aspects of the relationship between biotechnology and the environment. The papers are mainly devoted to the contribution of biotechnology in solving environmental problems, including biological waste water treatment, utilization of municipal sewage sludge, detoxification of polluted soil and complex utilization of lignocellulosic wastes. There is examination of possible dangers in such cases as the release of r-DNA organisms into the environment. The relationship of biotechnology and energy (e.g. biogas, landfill gas fuel, photosynthetic systems for fuel production) is also discussed.