No data at all has yet been published on therapeutic indices of any drugs in any form of schistosomiasis. Recent reports from this Laboratory (1, 2) have described methods for testing the effects of drugs on experimental schistosomiasis mansoni in mice. The drug testing was of a rough screening nature and only a very crude classification of drugs into four groups was provisionally attempted: 1. Drugs that cleared most of the mice of worms and left an average of not over one worm per mouse. 2. Drugs that left an average of between one and five worms per mouse, caused a shift of most of the worms to the liver and reduced the proportion of worms paired to values below fifty per cent. 3. Drugs that during treatment caused a shift of most of the worms to the liver and a breakup of most worm pairs followed after the end of treatment by a reversal of these changes, but that never caused the average number of worms per mouse to fall below five.