Pyrantel pamoate was administered as a single dose of about 5 mg per pound of body weight to 158 patients infected with helminths. An oral suspension of the drug was used to treat 69 patients infected with Ascaris lumbricoides, Necator americanus, or Trichuris trichiura, or more than one of these worms. The oral suspension and a chewable tablet were used alternately to treat 89 patients infected only with A. lumbricoides. Both preparations were palatable and well accepted, and side effects were few and mild. There was no evidence of toxic effects as measured by tests of renal and hepatic function and alterations of blood counts. Both formulations of the drug were equally effective in producing 30-day cures of infections with A. lumbricoides and N. americanus, but neither was effective against T. trichiura.