A double-site enzyme-linked lactate dehydrogenase enzyme immunodetection assay was tested against field isolates of Plasmodium falciparum for assessing in vitro drug susceptibilities to a wide range of antimalarial drugs. Its sensitivity allowed the use of parasite densities as low as 200 parasites/microl of blood. Being a nonisotopic, colorimetric assay, it lies within the capabilities of a modest laboratory at the district level.