The conversion of carboxy hemoglobin (HbCO) to cyanmethemoglobin (MHbCN) by Drabkin's reagent has been shown to be considerably slower than is the same reaction with oxyhemoglobin(Hb02). Conversion of Hb02 to MHbCN is complete in 10 min. whereas total conversion of HbCO requires 90-120 min. This difference in reaction rate must be considered in the determination of total hemoglobin as MHbCN when the blood contains appreciable HbCO.