We have previously reported the isolation and testing of a DNA probe specific for the detection of Plasmodium falciparum. Field studies to compare the sensitivity and specificity of the DNA probe with that of light microscopy have been performed. In 2 studies in Thailand, 1,397 patients were tested. Microscope slides were prepared in a standard fashion and examined by clinical technicians and expert microscopists. The DNA probe method compares favorably in sensitivity with routine microscopy, detecting parasite densities as low as 40 parasites/µl blood in the first study and, after modifications, 20–25 parasites/µl blood in the second. Modifications included the elimination of salt from the lysis buffer, increasing the pH of the lysis buffer, and use of nylon based hybridization membranes instead of nitrocellulose. The DNA probe method offers the advantage of a standardized procedure that can be used in a batchwise fashion on a large number of samples.