Density-related subpopulations of platelets were obtained by centrifuging platelets obtained from venous blood on a discontinuous gradient of arabinogalactan (Stractan). Four subpopulations were obtained in fractions of the gradient with densities less than or equal to 1.058, 1.065, 1.070, and 1.092 g/ml. The percentage of the total platelet population recovered in these various fractions in 5 patients with congenital storage pool deficiency who are deficient only in dense granules (delta-SPD) was similar to that observed in 8 normal subjects. In contrast, a shift in the density distribution towards a higher percentage of less dense platelets was observed in 3 patients (alpha delta-SPD) who are deficient in both alpha and dense granules. The results of these studies strongly suggest that alpha-granules are a major determinant of platelet density.