When N2 microbubbles are stirred in platelet-rich plasma, they cause a fall in the number of free platelets. Changes in the platelets ultrastructure during this interaction between gas bubbles and platelets have been studied by transmission and scanning electron microscopy. Platelets, and aggregates of platelets, adhere to the surface of the N2 microbubbles. This adhesion induces ultrastructural changes (shape change, pseudopod formation, granule centralization, fusion, and disappearance) that are similar to "classical" agonists like ADP, collagen, and thrombin. These ultrastructural studies further strengthen our previous contention that N2 microbubbles activate platelets in a way similar to these physiologic agonists, and show that the previously reported fall in the number of free platelets is due to platelet aggregation. Platelet aggregates are also present in the interstices between gas bubbles. Fixed N2 microbubbles have been demonstrated, and these can be broken like cracked eggs by means of the electron beam in the electron microscope. Possible mechanisms for activation of the platelets by the gas bubbles are: through ADP released from some few platelets; by diffusion of gas after bubble-platelet interaction; and through certain fractions of the plasma proteins and lipids in the bubble surface that may act as binding sites for the platelets and promote adhesion and spreading of the platelets over the surface.