Supravital staining with crystal violet of normal erythrocytes which had been exposed to hydrogen peroxide vapour, resulted in the appearance not only of Heinz bodies but also in an overall blue coloration of a proportion of the erythrocytes. Inhibition of catalase by aminotriazole or by sodium azide greatly increased the proportion of such ‘blue cells’. A close relationship was found between the percentage of ‘blue cells’ and the degree of lipid peroxidation as measured by malonyldialdehyde fo