Polylysine aggregates human blood platelets and, within a concentration range which is typical for a given preparation, can elicit the release reaction. No evidence for a relation between the active concentration range of different polylysine preparations and their structure or molecular weight could be established. ADP stimulates the release at ub- or superoptimal concentrations of polylysine. These results support the hypothesis that cell contact, here facilitated by the polycation, ADP, fibrinogen and calcium ions are required for the induction of the release reaction. Once these components are available in adequate amounts within a polylysine-induced aggregate, the release reaction will be propagated automatically throughout the aggregate.