The effectiveness of various drinking water treatment processes has been tested in laboratory and pilot scale experiments using neurotoxic and hepatotoxic cyanobacterial blooms and laboratory‐grown algal cultures as test material. The species involved belonged to the genera Microcystis, Oscillatoria, and Anabaena. Microcystis and Oscillatoria hepatotoxins were analyzed by high performance liquid chromatography. The Anabaena neurotoxin, anatoxin‐a, was determined using gas chromatography/mass spectrometry techniques. Toxicity was tested and verified by mouse bioassay. The laboratory study, made with freeze‐dried cyanobacteria, showed that toxins were not removed by conventional flocculation treatment procedures. The simultaneous addition of a small amount of powdered activated carbon with the flocculation chemical did not improve the removal of hepatotoxins significantly. Substantial reduction of toxins was achieved by granulated activated carbon filtration and by ozonization. Pilot plant studies supported the laboratory findings. Results of the slow sand filtration experiments suggested some removal of toxins.