The sensitivity, specificity and convenience of carrying out malaria diagnosis in acridine orange stained capillary tubes using a fluorescent microscope (the QBC system) was compared to screening for Plasmodia on conventional Giemsa stained thick smears. A dilution study revealed that the QBC is able to detect Plasmodia in as low a dilution as 5 organisms per ul. The QBC system was evaluated at a district hospital in Thailand. A preliminary study of 186 patients compared the QBC system to the routine malaria screening procedure (screening up to 30 microscopic fields on a thick smear). The sensitivity of the QBC was found to be 98.9% with a specificity of 94.4%. A second combined series of 465 febrile subjects were screened by thick smear and these results were compared to the QBC. 202 were positive for malaria on both QBC and thick smear. Sensitivity in this study was found to be 99.5% (202/203) and the specificity was 94.6% (248/262). When both series were combined, there were 14 QBC malaria positives that were not detected on thick smear, and 2 QBC malaria false negatives among the 651 patients studied. The parasite densities in these cases were between 10 and 320,000 organisms/microliters. The QBC system provided only a crude estimate of the level of parasitemia. The species of Plasmodia (P. falciparum and P. vivax) were correctly identified on QBC in 78% of cases.