Interactions of the dysfibrinogen Oslo I with platelets were investigated. This fibrinogen is a B beta-chain variant with faster than normal fibrin monomer polymerization. Fibrinogen Oslo I acted more efficiently in ADP-induced platelet aggregation, and bound to gel-filtered platelets with a higher affinity constant than did normal fibrinogen. At all concentrations more fibrinogen molecules became bound per platelet with the dysfibrinogen than with normal fibrinogen, both when the fibrinogens were tested separately or as a mixture using 125I or 131I to label the two types. At high concentrations this was probably due to ligand polymerization of the dysfibrinogen. These observations indicate that the increased cofactor function in platelet aggregation may be related to the increased affinity of the dysfibrinogen for the platelets.