Myelomatosis was diagnosed in a 64 year old man on the basis of a serum paraprotein band (type IgG lambda, 42 g/l), plasma cell infiltration of bone marrow, and multiple lytic lesions evident on skull x ray picture. Blood specimens taken into plain glass tubes showed bulky gelatinous clot formation with minimal clot retraction. Coagulation tests were significantly abnormal with an increase in thrombin time, prothrombin time, and reptilase time. The possibility that the paraprotein was interfering with fibrin production was investigated. The rate of fibrin monomer polymerisation (measured turbidometrically) was reduced in patient plasma compared with control plasma. Although purified fibrin monomer prepared from the patient's fibrinogen polymerised normally, the addition of purified paraprotein caused a dose dependent reduction in the rate of polymerisation. These results suggest that the paraprotein was impairing fibrin formation by inhibiting fibrin monomer polymerisation. After chemotherapy the paraprotein concentration decreased and the coagulation results returned to normal.