For selective ultrastructural localization of acid mucosubstance in rabbit and human platelets and megakaryocytes, bone marrow and buffy coat specimens were fixed with formalin, glutaraldehyde or osmium tetroxide, sectioned at 40 µ and stained with the Rinehart-Abul-Haj solution of dialyzed iron. In specimens from both rabbit and man, dialyzed iron staining was observed within nucleoids of the cytoplasmic granules (α-granulomeres) of platelets and megakaryocytes, on the outer surface of the plasma membranes of platelets and megakaryocytes and on the luminal surface of demarcation membranes of megakaryocytes. These results were obtained following any of the three fixation procedures, except when nucleoids failed to stain after glutaraldehyde fixation. For ultrastructural localization of pyroantimonate-precipitable cation, bone marrow and buffy coat specimens were fixed in Komnick's solution of potassium pyroantimonate and osmium tetroxide. In specimens from both species, antimonate deposits were localized within the dense bodies (5-hydroxytryptamine organelles) of platelets and within nucleoids of cytoplasmic granules of platelets and megakaryocytes. The dense bodies were well preserved in platelets fixed in a pyrophosphate-osmium tetroxide solution but were poorly, if at all, preserved by osmium tetroxide solutions containing other buffers.