Monoclonal immunoglobulins with affinity for platelets were detected in the blood of seven patients. Two of these had thrombocytopenia and non-Hodgkin's lymphoma (NHL). One patient had thrombocytopenia and possibly incipient NHL. The other four patients had pseudothrombocytopenia at the time of diagnosis but one of them developed NHL six years later. It is suggested that these monoclonal immunoglobulins may in some cases be associated with malignant lymphoma and that subjects presenting with these immunoglobulins should have a long term follow-up in order to elucidate the question whether or not lymphoma will develop.