A study was made of the effect on taeniasis of mebendazole, a drug known to be highly active against a variety of nematode species parasitic in humans. Thirty-seven patients infected with Taenia solium and four with Taenia saginata, some of whom had concomitant nematode infections, were treated with one of three dosages for 3 to 6 days. Within 48 hours of treatment most patients had eliminated long chains of intact, but apparently lifeless, proglottids. Assessment of results was made by monthly post-treatment observations for 4 to 5 months. The optimum effective dose of mebendazole appears to be 300 mg twice daily for 3 days.