This paper describes the effect of oral metrifonate, an organophosphorus cholinesterase inhibitor, on Schistosoma haematobium infections.The methodology of initial studies in hospital patients and 3 field trials in schoolchildren, using spaced doses ranging from 5 mg to 15 mg per kg of body-weight is detailed.The expected fall in plasma cholinesterase was confirmed. No major side-effects were encountered and minor symptoms were associated with high doses only.The optimum dose was 7.5 mg per kg, given once every 14 days or once monthly to a maximum of 3 doses if necessary. About two-thirds of patients were cured after 1 or 2 doses, but some 10% of these relapsed within 6 months of treatment. In 2 trials of this dose, cure rates of 71% and 79% were obtained.The authors conclude that metrifonate is a useful addition to oral schistosomicides for the treatment of S. haematobium infections in children, but suggest that more trials with spaced dose regimes are indicated and that further experimental studies are needed to define its mode of action.