The biochemical mechanisms of enzyme deficiency were investigated for two low activity G6PD variants, i.e. G6PD Mediterranean and G6PD Cagliari (a new variant). This study required the complete removal of leukocytes and platelets from blood samples and the use of sensitized procedures for assays of activity. G6PD Cagliari has a highly accelerated decay within circulating RBC and a near normal catalytic efficiency. The deficiency typical of G6PD Mediterranean (approx. 0.1% of the normal levels) is mediated by moderately reduced specific activity, while no clear decay was seen in mature RBC: however, the minute levels of activity that are observed in reticulocytes suggest that a strongly enhanced breakdown takes place during maturation of erythroid cells.