Sewage from clinical chemical laboratories is despite of its low chemical load one of the main critical effluents from hospitals, particularly with regard to their ecotoxicity and mutagenic effects. Only little data is available with respect to their environmental impact. Therefore, an inventory was carried out in clinical laboratories and the biological degradation and mutagenicity of liquid reaction residues from analysers used in clinical chemical routine laboratory diagnostics were investigated. The reaction mixtures were only partly biodegradable. Thereby the possibility of transfer into water bodies and for accumulation in the food-chain absolutely consists for a part of it. Moreover, the highly with test reagents and patient material contaminated liquid reaction residues from analysers show strong mutagenic effects which were only partial eliminated microbially. For a final environmental risk assessment of the strong contaminated reaction mixtures from clinical chemical analysers further investigations into persistence, biological accumulation and degradation should be carried out.