Abstract: This invention relates to a process for the production of optionally foamed polyurethane plastics, particularly shaped articles, using certain denatured biomasses as reactive fillers. The fillers in question comprise biomasses based on microorganisms and the derivatives and decomposition products of microorganisms, particularly biologically purified sludges, which have been deodorized and irreversibly denatured by reaction with isocyanates and/or carbonyl compounds and compounds capable of aminoplast and/or phenoplast formation.