The increase in light transmittance of aspirin-treated platelet-rich plasma caused by addition of non-saturating doses of ADP and, at earlier times, of adrenaline is correlated with formation of aggregates having a volume in the range 490-8580 fl. and containing 100-2000 platelets. The disappearance of single platelets and the formation of aggregates having volumes less than 490 fl. makes no significant contribution to the increase in light transmittance. Similar relationships are observed on addition of saturating doses of ADP and adrenaline except that the formation of aggregates larger than 8580 fl. contributes significantly to the initial phase of the increase in light transmittance and is more closely correlated with the overall change in this parameter.