Morphological alterations of platelets during reversible aggregation mediated by diamide are compared with those observed during irreversible aggregation. Diamide prevents cell fusion and the formation of loop-like structures which probably arise from fusing plasma membranes. Differences in the protein pattern of platelets after reversible and irreversible aggregation suggest a specific involvement of SH-proteins in irreversible platelet aggregation. The diamide mediated alterations of platelet aggregation behaviour are transitory, i.e. reversibly aggregated platelets respond normally to a new addition of aggregation inducers without and with diamide, respectively.