In situ biological degradation involves stimulation of the indigenous subsurface microbial population, by the addition of nutrients and an oxygen source (hydrogen peroxide), in order to promote degradation of organic contaminants present in soils and groundwater. The site selected for the demonstration of in situ biodegradation contains a mixture of inorganic and organic contaminants including electroplating wastes, chlorinated solvents, cresols, and chlorobenzenes. The soils are classified as gravelly clay loams and the hydraulic conductivity within the demonstration site varies from 0.11 to 9.26 ft/day. The design of the field demonstration involved computer modeling using a two dimensional nonsteady state model to determine the number and placement of injection and extraction wells. Stabilized hydrogen peroxide and specially formulated nutrients are introduced to the subsurface and the groundwater is circulated by pumping and gravity injection. Routine monitoring of chemical and biological parameters has been conducted during a 3-month operation period from June through September 1985. These results are presented and indicate that rapid transport and breakthrough of ammonia and phosphate has occured in areas of high hydraulic conductivity. The microbial population has adapted to the addition of nutrients and incremental increases of hydrogen peroxide and remains viable. Comparisons of performance and cost are also presented against more traditional treatment approaches.