The composting test method based on activated vermiculite is a comprehensive system for the assessment of the environmental impact of biodegradable plastics. It allows, in a single test, (i) the measurement of the mineralization of the polymer under study; (ii) the retrieval of the final polymeric residues and (iii) determination of the biomass (to make a final mass balance); (iv) detection of breakdown products of the original polymer. In this study it is shown that the vermiculite test method is also suitable to perform ecotoxicological studies. The Flash test is a method based on kinetic measurement of bioluminescence by Vibrio fischeri, and was applied to evaluate the toxicity of compost samples and vermiculite samples after the biodegradation of a polyurethane (PU) based plastic material. Toxicity was detected in vermiculite samples contaminated by 4,4′ diamino diphenyl methane (MDA), a toxic breakdown product released by the PU moiety, as shown by HPLC. On the other hand, neither toxicity nor the presence of MDA was detected in mature compost. A recovery experiment previously performed had shown a 10% MDA recovery yield from mature compost. The possibility of testing the ecotoxicity of extracts obtained from mineral matrix after biodegradation makes the vermiculite test system particularly interesting for the overall assessment of the environmental impact of biodegradable plastics.