The oxy-Hb content of blood can be rapidly and accurately estimated with the help of a photoelectric colorimeter ("Absorptiometer," made by Adam Hilger Ltd., London). In whole blood the presence of metor of sulph-Hb or of other pigments can be detected by taking readings with 2 different color filters. The use of 0.4% ammonia as a diluent for blood for spectro-photometric measurements is not safe. Better results are obtained with a conc. of 0.04% ammonia.