The role drug resistance plays in the occurrence of chronic and recurrent giardiasis has not been established. Extensive data on the susceptibility to antimicrobial agents of living Giardia spp. trophozoites from human origin are lacking. We have determined with a macrodilution method in semisolid medium the in vitro susceptibility of 25 Giardia lamblia isolates, all obtained by routine cultivation of the duodenal fluid of children to six commonly used antiprotozoal drugs. The results showed tinidazole to be the most active drug (all isolates have MICs of less than or equal to 0.5 micrograms/ml). Metronidazole was equally active on all but one isolate, for which an MIC between 0.5 and 1 micrograms/ml was found. Furazolidone was the most active nonimidazole compound tested. More than 50% of the isolates were very susceptible to paromomycin, pyrimethamine, and chloroquine. Two of the strains presented an MIC for paromomycin higher than 10 micrograms/ml, and six strains needed more than 50 micrograms of pyrimethamine per ml to be inhibited. Decreased susceptibility of several of the isolates to different agents appears to be linked.