Normal human platelets are characteristically discoid in shape. The lentiform appearance is supported by a circumferential band of microtubules lying just under the cell membrane along its greatest circumference. Some of the cells from patients with giant platelet disorders are also disk-shaped, but the majority of their huge platelets are spherical. In the present study platelets from patients with the Gray platelet syndrome (GPS), May-Hegglin anomaly (MHA), and Epstein's syndrome (ES) were examined in thin sections and negatively stained whole mounts, and by indirect immunofluorescence with a monoclonal antibody to tubulin for determination of the organization of their microtubule systems. Many GPS platelets and some ES and MHA platelets were discoid and contained circumferential bundles of microtubules. The number of coils in the band was increased 10-20-fold. Giant spherical platelets also contained increased numbers of individual microtubules and coils, but they were not organized into circumferential bundles. Immunofluorescence revealed an organization of assembled tubulin in the huge cells, suggestive of balls of yarn. Failure of the microtubules to organize into a circumferential band may explain why the majority of the huge cells have a spherical form.