The cyanobacterium Cylindrospermopsis raciborskii produces a toxin known as cylindrospermopsin. This toxin has been implicated as the causative agent of the Palm Island Mystery disease which led to the hospitalisation of 150 people in Queensland in 1979. More recently this cyanobacterium and other cylindrospermopsin-producing cyanobacteria have become increasingly of concern. They have been found in high numbers in drinking water reservoirs such as those supplying Brisbane. C.raciborskii has also been found in the more temperate waters of the Murray-Darling basin and in European freshwaters. It is known that cylindrospermopsin is a potent inhibitor of protein synthesis. We have utilised the rabbit reticulocyte lysate system to assay for protein synthesis inhibition. The response of cylindrospermopsin in this system has been compared to cycloheximide, a well known and widely used inhibitor of protein synthesis. Samples of C.raciborskii extract have been assayed for cylindrospermopsin content and the results correlate well with High Performance Liquid Chromatography (HPLC) data. We expect that the protein synthesis inhibition assay will be a useful tool for the rapid assessment of the bioactivity of C.raciborskii samples, as has been the case in our laboratory.