Lipopolysaccharides (LPS) are produced by Gram negative bacteria including cyanobacteria. Bacterial LPS and their biological activities are well studied in relation to mammals, with particular reference to human medicine. Cyanobacterial LPS are less well studied and though they may be of lower toxicity to rodents than the LPS of Salmonella , they can be found in large quantities in cyanobacterial blooms. Laboratory purification by hot phenol extraction suggests that LPS may account for between 0.1-1% of cell dry weight of an axenic culture, and that toxicity according to Limulus assay varies between strains. Natural blooms are not found under axenic conditions, often they have bacteria associated with them, including coliforms. Commercially available Salmonella and E.coli LPS and three cyanobacterial LPS‚ (1 laboratory culture, 2 natural blooms) were used in the investigations. All LPS preparations strongly inhibited microsomal glutathione S-transferase (GST; EC 2.5.1.18) in zebra fish p6 stage embryos in vivo: enzyme activity was reduced from 0.502 to between 0.064 and 0.315 nkat.mg-1 protein. Soluble GST showed no inhibition in response to Salmonella or E.coli LPS, however activity was significantly reduced from 1.045 to between 0.186 and 0.216 nkat.mg-1 protein after exposure to all cyanobacterial LPS. When exposed to both MC-LR and LPS the microsomal GST activity in all exposures was reduced, soluble GST activity was reduced only in the cyanobacterial LPS-treated embryos. The results will be discussed in the context of microcystin detoxication and possible implications for fish and animal health.