A cytochemical method has been described for the specific and direct staining of glucose-6-phosphate dehydrogenase (G-6-PD) in red blood cells (RBC), by using the tetrazolium salt Nitro-BT (NBT). This method allows a semiquantitative estimation of G-6-PD activity in single cells. The results of such estimations were found to be correlated with spectrophotometric determination of G-6-PD activity in RBC lysates. The activity of G-6-PD was shown to decline with aging of the cell. In cases of severe G-6-PD deficiency, where the spectrophotometric assay revealed no activity, staining of ‘young’ RBC showed appreciable activity, while the ‘old’ cells showed no activity.