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1. An inhibitory action of the mitosis inhibitors colchicine, colcemid and vinblastine on blood platelet aggregation has been demonstrated.

2. The inhibitory action of low concentrations of these drugs was surmountable by increasing the concentration of the aggregating agents adenosine diphosphate (ADP) or thrombin. High concentrations completely inhibited aggregation.

3. The presence of a circumferential bundle of microtubules has been demonstrated in rabbit platelets.

4. After exposure to aggregating doses of ADP and thrombin the microtubules move to the centre of the cell, `trapping' the platelet intracellular organelles in a relatively clear cytoplasm.

5. Concentrations of the antimitotic agents which reduce platelet aggregation depolymerize the platelet microtubules and prevent the rearrangement of the platelet organelles during aggregation.

6. It is suggested that the microtubules are not essential for platelet aggregation but are intimately involved in the movement of intracellular granules. This may be a preliminary stage in the eventual discharge of the granules during the platelet `release reaction'.