The present case study develops and applies a systematic approach to the precautionary pre-screening of xenobiotic organic chemicals with respectto large-scale environmental threats. It starts from scenarios for uncontrollable harm and identifies conditions for their occurrence that then are related to a set of amplifying factors, such as characteristic isotropic spatial range p. The amplifying factors related to a particular scenario are combined in a pre-screening filter. It is the amplifying factors that can transform a potential local damage into a large-scale threat. Controlling the amplifying factors means controlling the scope and range of the potential for damage. The threshold levels for the amplifying factors of each filter are fixed through recourse to historical and present-day reference chemicals so as to filter out as many as possible of the currently regulated environmental chemicals and to allow the economically important compounds that pose no large-scale environmental concern. The totality of filters, with each filter corresponding to a particular threat scenario, provides the filter series to be used in precautionary regulation. As a demonstration, the filter series is then applied to a group of nonreferential chemicals. The case study suggests that the filter series approach may serve as a starting point for precautionary assessment as a scientific method of its own.