The activity of glucose-6-phosphate dehydrogenase (G-6-PD) in leucocytes was studied in the following group of Greek people. Group 1: 43 male children and 16 male students with mean values of enzyme activity of 27.7 +/- 16.6 units and 24.6 +/- 5.6 units, respectively. Group 2: 15 G-6-PD deficient male children who had never experienced an acute haemolytic episode with a mean value of 10.8 +/- 4.6 units. Group 3: 19 G-6-PD deficient male children during favism and 3 months after the haemolytic crisis with mean values of 8 +/- 4 units and 9.2 +/- 1.9 units, respectively. Group 4: 19 mothers of children from group 3 who by definition were carriers of G-6-PD deficiency had a mean value of 18.2 +/- 8.2 units. The difference between means for group 1 and groups 2, 3 and 4 is highly significant (P < 0.001). Therefore the enzymatic defect in Greek people is not limited to the erythrocytes but can be also demonstrated in leucocytes.