Platelets have a characteristic lenslike appearance in circulating blood. In vitro studies have shown that the discoid shape is supported by a circumferential band of microtubules. Early studies demonstrated that chilling caused platelets to lose their disklike form and become irregularly convoluted with multiple pseudopods. The cold-induced shape change was associated with disappearance of the annular band of microtubules. Rewarming caused reassembly of the circumferential bundle and recovery of platelet discoid form. The present study has examined the influence of taxol, a microtubule stabilizing agent, on the response of platelets to low temperature. Taxol-treatment protected platelet microtubules from disassembly in the cold and preserved the discoid shape of most platelets. Addition of taxol to platelets prechilled to remove microtubules and maintained in the cold resulted in assembly of tubular polymers at low temperature. Brief exposure to taxol in the cold did not prevent recovery of platelet discoid shape on rewarming to 37 degrees C. However, the bundle of tubules was often located in the central axis, rather than in a circumferential band. Longer incubation with taxol at low temperature resulted in assembly of radiating bundles of tubules. On rewarming, these cells remained irregular in form and did not develop circumferential bands.