A rapid extraction and purification procedure is described for the preparation of toxic peptides from freshwater blooms and laboratory isolates ofMicrocystis aeruginosa. Extraction with methanol/butanol, followed by C18cartridge concentration; gel filtration and high performance liquid chromatography yields discrete toxin peaks. Elution profiles for the laboratory isolates and bloom extracts are compared and the applicability of the method for detecting cyanobacterial toxins in natural waters is discussed.