Washed platelets were ruptured by freezing and thawing; a coagulant activity was released which would correct the clotting time of factor XI (FXI)-deficient plasma only in the presence of kaolin. Platelets from a FXI-deficient patient treated in a similar fashion also released a coagulant activity which could be absorbed onto Sepharose-heparin and eluted similarly to plasma FXI.Collagen was employed to induce a coagulant activity in platelet-poor plasma (PPP) and platelet-rich plasma (PRP) in the presence and absence of antibodies developed to purified FXI and FXII. The presence of FXII antibody had little effect on the activity induced in PRP. However, the presence of FXI antibody eliminated the difference between PPP and PRP. An activity was induced when FXI-deficient PRP was incubated with collagen and none with PPP. One type of collagen failed to induce a coagulant activity.