This paper outlines the experimental design and techniques used in the initial multicentre clinical experiences with praziquantel in the treatment of human infections due to Schistosoma haematobium, S. mansoni, and S. japonicum. Trials were conducted in Brazil, Japan, the Philippines, and Zambia.Close professional cooperation between informed representatives of the manufacturers of the drug and WHO led to the use of a standard clinical trial design and agreed technical protocols, although parasitological methods of therapeutic assessment varied with the species of infecting parasite. Double-blind studies of tolerance were conducted at three different dose levels and subsequently, in Brazil and Zambia, single-blind trials of parasiticidal efficacy were carried out. The results of the various trials are reported separately.This type of close professional cooperation is a useful model for initial clinicopharmacological studies of parasiticidal drugs-an area beset with difficulties for both industry and international agencies.