This document specifies the procedure for preparing natural seawater and seafloor sediments used in laboratory-scale test methods to evaluate the degree and rate of aerobic biodegradation of plastic materials in marine environments under aerobic conditions. The methods described are designed to collect filtered seawater, washed seawater and sediments in the sand-gravel region from natural seawater and seafloor sediments in the intertidal zone using wet filtration and flotation, improving the reproducibility of test results when determining the biodegradation of plastic materials. This document does not include steps to improve the biodegradation of plastic materials by concentrating the natural seawater, adding nutrients to the seawater, and pre-culturing the inoculum. The methods described in this document are intended to be used in addition to issued ISO standard test methods for evaluating the biodegradation and disintegration of plastic materials. The applicable evaluation test methods are ISO 18830, ISO 19679, ISO 22404, ISO 23977-1, ISO 23977-2 and ISO 23832. The conditions described in this document do not always correspond to the optimum conditions for maximum biodegradation.