The recent rise in the awareness of the occurrence of toxic cyanobacterial blooms in aquatic environments, with associated human health problems and animal deaths, has increased the need for rapid, reliable and sensitive methods of determining cyanobacterial toxicity. A luminescent bacterial toxicity test was assessed as a complement to the established mouse bioassay. Seventeen samples including pure cyanobacterial microcystin-LR hepatotoxin, laboratory isolates and natural blooms of cyanobacteria were tested and toxicity data compared with mouse LD50 values. Microcystin-LR and all five microcystin-containing cyanobacterial samples, hepatotoxic by mouse test gave EC50 values of less than 0.46 mg/ml in bioluminescence-based Microtox assays. Of 11 samples non-toxic by mouse bioassay, only two gave an EC50 of less than 0.98 mg/ml by bioluminescence assay. It is suggested that the Microtox bioluminescence assay may prove useful in the preliminary screening of cyanobacterial blooms for microcystin-based toxicity.