Phases I and II (early and advanced) of platelet aggregation could readily be followed and studied by various topo-optical reactions. Our studies indicate that in the early stage (monitored by an aggregometer) aggregates show no alteration in membrane structure of platelet. In the advanced stage (phase II), in addition to membrane alteration, platelets 2–3 times larger than in the control were observed. These were stained selectively in the pH 1–2 range with thiazine dyes. After hyaluronidase digestion they lost the ability to stain and became birefringent. The elucidation of this phenomenon requires further study.