The diazo method of Hunter for the determination of ergothioneine in simple solution has been adapted to the Evelyn photoelectric colorimeter. The application of the method to the determination of ergothioneine in blood is described. Suitable blood filtrates are prepared by deproteinization with standard acetic acid oxalate solutions followed by removal of reduced glutathione with Goulard's lead acetate solution, and removal of excess lead with phosphate. By the method precise recovery is obtained of ergothioneine added to human blood.