Fibrinogen in aqueous solution is precipitated by the antibiotic ristocetin. This reaction is inhibited by albumin and facilitated by low temperature. Resolubilized fibrinogen clots in the presence of thrombin. Ristocetin-precipitated fibrinogen takes the form of fibrils or clumps, composed of irregularly spaced, structure-less particles. The addition of ristocetin to washed platelets suspended in fibrinogen- containing media produces fibrinogen clumps in both the media and in the surface cannalicular system of the platelets. The changes in light transmission (aggregation curves) are due to both platelet aggregation and fibrinogen clumping. The role of the latter is confirmed by the observation that the addition of ristocetin to inert latex particles suspended in fibrinogen solution produces typical aggregation curves. This phenomenon is prevented by the addition of albumin to the media. We conclude that (1) if fibrinogen is present in any artificial system, albumin should be included in the media to prevent fibrinogen precipitation; and (2) statements about aggregation of any particulated materials by ristocetin should not be based solely on light- transmission changes, but should also include a description of the morphologic appearance.