An easily performed reference haematocrit would be useful in quality control, in the preparation of whole-blood calibrators and in the evaluation of automated, whole-blood analysers. Available techniques are complex, time-consuming and require radiolabelled reagents that are difficult to obtain. They have largely been directed towards correction of 'trapped plasma' errors. Other errors which affect haematocrit measurement but which have not yet been addressed include those due to white-cell contamination, those due to defects in tube plugging and those resulting from red-cell dehydration. We propose a simple, rapid and reliable reference haematocrit procedure which relies on the fact that haematocrit equals haemoglobin/MCHC and that MCHC determinations via haemoglobin analyses can be performed accurately on as little as 6 microliters of packed red cells (CV less than 0.5%). Selection of the cells to be analysed from the mid portion of a packed red-cell column eliminates interference from white-cell contamination/red-cell dehydration. The absence of trapped plasma in this portion of the red cell column after centrifugation for 4 min at 10,000 r.c.f. can be documented by a second cycle of centrifugation.