A rapid diagnostic test for malaria based on acridine orange staining of centrifuged parasites in a microhaematocrit tube ('QBC' tube) was compared with the thick blood smear in 12 volunteers experimentally infected with Plasmodium falciparum, 408 residents of a malaria endemic area, and 180 hospital patients with suspected malaria. In the experimentally infected volunteers, the QBC tube test and the thick blood smear were comparable and the QBC tube could detect as few as 4 parasites/microliter blood. When used for mass screening in the field study, the test had a sensitivity of 70% for the diagnosis of malaria compared with 92% for a single thick blood smear. However, when used to diagnose malaria in hospital patients, the test detected as few as 3 parasites/microliter in 91 of 92 patients with asexual parasitaemia. For the three studies, the QBC tube was highly specific (98.4%), indicating malaria in only 8 of 487 subjects with negative blood films. The species of parasite was correctly identified in 77% of species. Processing the QBC tube was easier and much more rapid than was processing a thick blood smear, taking only 5 min for centrifugation and 5 min for examination. The QBC tube is not a substitute for the blood smear, but its speed and ease of use make it an important new tool for the diagnosis of malaria.