This study of 20 patients with monoclonal gammopathy (17 patients with multiple myeloma and 3 patients with Waldenström's syndrome) showed a decreased platlet aggregation induced by ADP and collagen, a reduced platlet retention and a prolonged bleeding time in 25% of the cases in comparison with 30 age- and sex-matched healthy controls. Using Wu and Hoak's technique as increased number of reversible platelet aggregates was observed. There was no relationship between the parameters of primary haemostasis and protein analysis. A disturbed mechanism of primary haemostasis is the main factor responsible for the bleeding diathesis in patients with monoclonal gammopathy.