The Micro-Methemoglobin Reduction Test, when used as a field screening procedure, identifies males who are glucose-6-phosphate dehydrogenase deficient. It does so without a requirement for temperature control or accurate timing. The end point is determined visually. By the use of temperature and time control, the Micro-Methemoglobin Reduction Test can also be used as a semiquantitative procedure. In this case the endpoint is determined by measurement of methemoglobin at the end of the incubation. This procedure allows the detection of a considerable portion of heterozygous females. The test is performed in a previously prepared capillary tube containg dried reagents. It requires only a small quantity of blood; enough blood for the test can be obtained by fingerprick.