effect of phosphonium salts and phosphoranes on the acetylcholinesterase activity and on the viability of schistosoma mansoni parasites in this study we investigated the effects of a series of phosphonium salts and phosphoranes on the catalytic activity of acetylcholinesterase and on the viability of the various life stages of schistosoma mansoni worms all the tested compounds showed an inhibitory effect towards the s mansoni acetylcholinesterase ache activity the most effective compound pxylylene bistriphenylphosphonium dibromide no 16 displayed approximately 100 inhibition at concentration of 105106m no significant difference was found in the sensitivity of the enzyme obtained from the various stages of the parasite life cycle to the effect of the drugs each compound was also tested for its toxicity towards 3 h old schistosomula and 79 week adult worms under in vitro culture conditions in the case of the larvae after 2 days in culture only three compounds nos 4 11 and 12 out of sixteen tested exhibited efficient killing of the schistosomula while the others had a very slight toxic activity or no toxicity at all on the other hand all the compounds showed a significant toxicity towards the adult worms and the most effective one allytriphenylphosphonium bromide no 11 retained its toxic effect even at an extremely high dilution 108m however the cumulative results in this paper do not demonstrate a significant correlation between the inhibitory effect of the phosphonium salts and phosphoranes on the ache activity of the schistosomes and their toxicity towards the worms the ld50 value iv of the compound which showed the highest toxic effect in vitro no 11 was found to be 30 17 mgkg in miceabstract truncated at 250 words