A sensitive and rapid colorimetry for determination of methemoglobin (MetHb) in hemolysate of the test blood was devised by measurement of absorbance at 563 nm, the isosbestic point of spectra of cyanomethemoglobin and oxyhemoglobin, at pH 6.8. MetHb was determined as the difference in absorption caused by cyanide in the absence of potassium ferricyanide divided by the difference in absorption caused by cyanide in the presence of the ferricyanide. Carboxyhemoglobin (COHb) in the blood was also estimated from the absorbance values of the hemolysates with or without potassium ferricyanide after the addition of cyanide. The method requires only 3 microliters of test blood and 10 min for determinations of MetHb and COHb in blood. Results obtained by the method were in satisfactory agreement with theoretical results for mixture of MetHb, COHb, and oxyhemoglobin. The method was compared with two other methods using 55 forensic blood samples containing various amounts of MetHb and COHb, and proved to be suitable for practical samples.