SummaryThe frequency distribution of platelet sizes in platelet populations of various ages was investigated using a Coulter Electronic Particle Counter, model B, and a particle size distribution plotter. Platelets were obtained from male Sprague-Dawley rats, young platelets from rats recovering from thrombocytopenia induced with platelet-specific antiserum, and old platelets after 2 days' in vivo incubation of normal platelets in irradiated, thrombopenic rats. The size distribution curves demonstrated a shift toward larger than normal size in the case of young platelets and smaller size for old platelets. Statistical analysis of the curves gave unbiased estimates of mean sizes and variances which were used to test hypotheses concerning the effect of age on platelet size.