The presence of human fibrinogen in suspensions of washed human platelets is a requirement for ADP-induced platelet aggregation. Digestion of fibrinogen with plasmin destroys this function of the protein. The high solubility fraction of Kabi fibrinogen, fragment X (stage 1) and framgent X (stage 2), are two, eight, and ten times, respectively, less potent in promoting ADP-induced platelet aggregation, as compared with intact fibrinogen. Fragments Y and D and the mixture of reduced and carboxymethylated chains of human fibrinogen do not support ADP-induced platelet aggregation at all. SDS polyacrylamide gel electrophoresis of nonreduced and reduced fibrinogen and its derivatives indicates that the intact fibrinogen molecule is essential for ADP-induced platelet aggregation. It is suggested that the carboxy-terminal part of the Aalpha chain and possibly also the amino-terminal part of the Bbeta chain are required for the platelet aggregation-promoting function of fibrinogen.