Abstract: When male worms of Schistosoma japonicum were maintained in culture medium containing immune rabbit serum (IRS) and exposed to pyquiton, numerous granular flocculous materials deposited on the drug-damaged surface of the worms. The flocculous materials aggregated together to form a membrane-like sheath around the worm accentuating the tegument injury. If IRS in the medium was replaced by normal rabbit serum (NRS), no such phenomenon was observed. When male schistosomes were exposed to pyquiton at a concentration of 30 μg/ml for 1 or 4 h, then transferred to the medium containing NRS and incubated for another 3 days, the damaged tegument and worm activity recovered apparently in about one half of the worms tested. However, if the male schistosomes were transferred to the medium containing IRS, most or all of the worms showed no significant recovery. In another experiment, male schistosomes, perfused out from infected mice 4～8 h after oral administration of pyquiton at a single dosage of 400 mg/kg, were transferred to the medium containing IRS and incubated for another 3 days, no apparent recovery of tegument damage nor worm activity was detected in most or all of the worms tested, while transferred to the medium containing NRS exhibited recovery in different degree. The results were also confirmed by scanning electron microscopic observation.