The WOO test, or Haematocrit Centrifuge Technique (HCT), is the technique most commonly used for diagnosis of animal trypanosomiasis, but its sensitivity is not well defined. According to the authors, the sensitivity of the MURRAY technique (dark ground burly coat method, or DG/BCM) is higher, equal or lower than that of the WOO test for the detection of Trypanosoma vivax. Most authors have compared particular techniques relative to other techniques, but not relative to a fixed reference. The relative sensitivity of particular tests appears to vary between technicians. The objective of the present study was to measure the sensitivity of the WOO test for T. vivax detection, using blood samples with pre-determined levels of parasitaemia, ranging from I to 1767 parasites/ml, that were prepared by mixing infected ovine blood with non infected ovine blood. A simple technique is described for the enumeration of parasites in the blood. The mean positivity level of WOO test was about 200 +/-100 T. vivax /ml. The sensitivity of the test was 100% above 700 parasites/ml; 80% between 300 and 700; 50% between 60 and 300, and it was negligible below 60 parasites/ml. Factors are provided to quantify parasitaemia relative to the number of parasites observed between microscope slide and cover slip (parasitaemia > 2000) or in the capillary tube (parasitaemia < 2000). Sensitivity of the techniques for detection of active infection could be evaluated in reference to fixed values such as known parasitaemias, artificially created, as described.