Cytochemical localization of G6PD activity in erythrocytes from the patient with G6PD Volendam and her family. The test was performed as described.12 G6PD activity in individual erythrocytes was analyzed as production of formazan by reduction of tetra NBT and quatitated by means of cytophotometry through a microscope. The mean integrated absorbance per cell was measured in 50 randomly chosen cells, corrected for aspecific light loss in cells incubated in the absence of glucose 6-phosphate and expressed in activity grades from 1 to 10 (grade 1 = 0 to 10 arbitrary units, grade 2 = 10 to 20 arbitrary units, etc, up to grade 10 = 90 to 100 arbitrary units). (A) Histochemical localization of G6PD activity in erythrocytes from a healthy control, the father and mother of the patient, and the patient herself. Original magnification x 400. (B) Quantitative scores of these reactions are shown for the control, the father, the mother, and the patient.