Eleven minor components were isolated, together with microcystin-LR (LR, 1, Scheme I) as the principal toxin (ca. 90% of the toxic components), from Microcystis cyanobacteria (blue-green algae) collected from Homer Lake (Illinois) in the summer of 1988. The components were characterized by amino acid analysis and HRFABMS, FABMS/MS, ¹H NMR, and UV spectroscopic methods as microcystins-RR (2) and -YR (3) (Scheme I) and nine new microcystins. The structures of seven new microcystins were assigned as [DMAdda⁶]microcystin-LR (4), [Dha⁷]microcystin-LR (5), microcystin-FR (6), microcystin-AR (7), microcystin-M(O)R (8), [Orn⁷]microcystin-LR (9), and microcystin-WR (12). Compound 4 is the first microcystin containing 9-O-demethyl-Adda, while phenylalanine, N-methylserine, and tryptophan are also new variations in amino acid components of microcystins. Compound 11 was deduced to be a (C₃H₆O) monoester of the α-carboxyl on the Glu unit of LR (1). New microcystin 11 caused no apparent toxic effects in mice dosed ip at 1 mg/kg, while the others had LD₅₀'s of 90-800 μg/kg. The microcystins are well-known cyclic heptapeptide hepatotoxins obtained from cyanobacteria (blue-green algae), which grow worldwide in fresh and brackish waters and cause animal and human water-based toxicosis. Nine chemically defined microcystins (1-3 and 13-18, Scheme I) have been isolated from the genera Microcystis, Anabaena, and Oscillatoria. Microcystis is the most common producer of these hepatotoxins, and microcystin-LR (LR, 1, Scheme I) occurs most often. The structures of the microcystins differ primarily in the variations in the two amino acids at positions 2 and 4 and secondarily in the absence of the methyl groups on D-erythro-β-methylaspartic acid (D-MeAsp) and/or N-methyldehydroalanine (Mdha) (Scheme I). Nodularin (19, Scheme I), isolated from Nodularia spumigena, is thus far the only related cyclic pentapeptide, and it possesses similar hepatotoxicity. Hepatotoxic Aphanizomenon and Gomphosphaeria species have also been reported. The recently reported inhibition of protein phosphatases 1 and 2A by these toxins makes them important biological tools. The most unusual feature of nodularin and microcystins is the C₂₀ amino acid, (2S,3S,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (Adda), which plays an important role in their toxicity. Hydrogenation or ozonolysis of the diene system in the Adda unit gives an inactive product, and the stereoisomer at the Δ⁶ position of Adda reduces toxicity.