The glycogen content of human megakaryocytes was studied using a quantitative method. Smears of bone marrow from 13 individuals were stained with the modified PAS reaction with and without prior treatment with alpha-amylase. The intensity of the reaction was determined by microspectrophotometry in 50 megakaryocytes from each individual. It was found that megakaryocytes are rich in glycogen which is not only confined to the intensely PAS-positive granules and inclusion bodies, but also makes up a good part of the diffuse cytoplasmic staining. In the diffusely stained megakaryocytes, glycogen makes up 32% of the intensity of the PAS reaction, while it reaches 47% in those with granules and up to 60% in those with inclusion bodies. The total extinction of the PAS-stained megakaryocytes is not only dependent on the morphological appearances of the cells, but is also positively correlated with their size.