cyanobacterial blooms in human water supplies have been associated with increased liver enzymes in serum and conventional water treatment procedures may not effectively remove the hepatotoxins produced by microcytis aeruginosa m. aeruginosa, a fresh water cyanobacterium, produces cyclic heptapeptide hepatotoxins termed microcystins (and sometimes cyanoginosins) which cause livestock and wildlife poisonings world wide, especially following ingestion of cells that accumulate along the leeward shore of ponds and lakes in summer and early fall (carmichael et al., 1985). the recent development of improved purification methods for the microcystins aided rinehart et al., (in press) in their elucidation of the absolute structure of microcystin-lr (mclr), a member of the microcystin family. death following microcystin administration to rats and mice was attributed to extensive hepatic hemorrhaging causing hemorrhagic shock. microcystins accumulate in the liver after administration of a lethal dose (falconer et al., 1986) and exert an effect on the hepatocyte cytoskeleton. despite the extensive knowledge base gathered on the microcystins in the past 40 years, there is little published data on the effects that a sublethal dose would have upon a subsequent lethal dose of these remarkably potent hepatotoxins