AbstractSpecific, straightforward, and rapid procedures are required for the detection, identification, and quantification of the potent low molecular weight toxins that are produced by blooms and scums of cyanobacteria (blue‐green algae) in waterbodies.Use of the Microtox bioluminescence assay and the brine shrimp (Artemia salina) has been advocated for the initial screening of cyanobacterial blooms for microcystin hepatotoxins. Inhibition of bacterial luminescence in the Microtox assay and brine shrimp mortality were determined with microcystin‐containing and nonmicrocystin‐containing cyanobacteria.Extraction and fractionation of test samples was undertaken to select and isolate microcystincontaining fractions and reduce interference from other fractions. Maximal inhibition of bacterial luminescence in the Microtox assay occurred with fractions fromMicrocystisstrains and anAnabaenabloom that did not contain microcystins. By contrast, the bioassay of fractions using brine shrimps correlated with the distribution of microcystin‐LR in the fractionatedMicrocystisextracts. ©1994 by John Wiley&So