Platelet activation is accompanied by changes in the composition of the platelet cytoskeleton with rapid incorporation and displacement of certain proteins. Here we have inhibited cytoskeletal assembly by pretreating platelets with cytochalasin D (CyD) and investigated the effect on the stability of the aggregates that form. The experiments were performed in both citrated and hirudinized platelet-rich plasma (PRP) and aggregation was induced by adenosine diphosphate (ADP), collagen, the TXA2-mimetic U46619 and adrenaline. Platelets in the aggregates that formed, underwent rapid disaggregation on addition of EDTA or a GpIIb-IIIa antagonist such as MK-852 and GR144053F, all of which are agents that interfere with the ability of fibrinogen to interact with GpIIb-IIIa. This was the case irrespective of the aggregating agent used and occurred in both citrated and hirudinized PRP. In contrast, the rate of disaggregation brought about by some other agents, iloprost and ARL 66096, appeared to be unaffected by CyD. Information was also obtained on the effects of CyD on the cytoskeletal changes brought about by ADP and the effects on the cytoskeleton of subsequent addition of M K-852. The results show that CyD retards the incorporation of certain proteins (actin, myosin, alpha -actinin, actin binding protein and a 66 K protein) into the cytoskeleton and that subsequent addition of MK-852 results in rapid displacement of some of these with re-incorporation of a 31 K protein. The results suggest that the early changes in the cytoskeleton following platelet activation contribute to the stability of the aggregates that form, and that interference with these early changes results in aggregates that are easily disassembled by agents that interfere with GpIIb-IIIa-fibrinogen complex formation.