A simple spectrophotometric method of standardization of platelet suspensions is described which demonstrates an initial gradual change in optical density for 4 days followed by a stable light transmittance for up to 2 years. Hence standardization is possible at any suspension age. Several series of microcomplement‐fixation tests are presented which characterize the slow deterioration of antigenic strength of these suspensions, with good reactions still obtained on suspensions almost 2 years old when strong antisera are investigate