The use of a photoactivated, computerized monitor of schistosome motility is described. The effect of antimony tartrate, niridazole, and hycanthone on parasite activity in vitro has been determined. This effect was measured both directly and following stimulation with serotonin. It was found that accurate dose response curves to three common anti-schistosome drugs could be obtained. These show that antimony tartrate and niridazole suppress worm motility at doses from 10-3 to 10-5 M concentration. Hycanthone decreases worm activity at high dosage (10-3 to 10-4 M) but has a strongly stimulatory effect at 10-5 to 10-7 M concentration. The possibility that hycanthone acts by virtue of serotoninergic or anticholinergic functions is discussed.