Blood platelets normally circulate in the form of flat discs. Under conditions which promote aggregation, platelets transform from discs to irregular spheres and become sticky. Exposure of platelets to low temperature for brief periods causes similar shape changes. The present study demonstrates that the alterations induced in platelets by chilling are related to disappearance of a marginal bundle of microtubules, which has recently been described in platelets. Rewarming platelets which have previously been chilled results in recovery of both the marginal band of tubules and platelet disc shape. Thus the bundle of microtubules in platelet hyaloplasm appears intimately associated with disc shape of unaltered cells. Factors which influence platelet shape changes may do so by acting on the supporting marginal band of microtubules.