A quantitative complement-fixation technique for the detection of human platelet iso-antibodies and antigens is described. The method is based on the qualitative technique of Aster et al. . The results are read photometrically,which permits interpretation of reactions too weakly positive to be recognized with certainty visually, and allows quantitative investigations of the platelet-platelet antibody reactions. The ability to fix complement with the same amount of antibody was found to be better in platelet suspensions one and two days old than in suspensions prepared on the day of testing. The antigens were apparently stable for several months at 4°C, and one-year-old platelet suspensions could still be typed. Freezing to -20°C and to -80°C reduced the reactivity of platelets, and storage at-20°C caused a progressive loss of activity. The platelet concentrations yielding the highest serum titres varied between 17,000 and 117,000 platelets per μl of the reaction mixture in different experiments, and the ,antigen cut-off’, when present,occurred with from 3000 to 35,000 platelets per μl of the reaction mixture. These figures express differences in antigen content of different platelets. It is suggested that part of these differences is due to gene-dose effects. The practical applications of the method within histocompatibility testing and platelet immunology are briefly discussed.