WHEN washed suspensions of mammalian red cells are treated with isotonic solutions of nitrites, the iron of their contained hæmoglobin is oxidized to the ferric state with the formation of brown methæmoglobin. Using standard strength red cell suspensions and weak solutions of nitrite, the resistance (reaction time, Figs. 1â 4) of the hæmoglobin to this change can readily be measured¹.