The intracytoplasmic inclusions seen in most cells from a patient with B prolymphocytic leukaemia were analysed using both light and electron microscopy. They consisted of a dense homogeneous structure and were surrounded by a membrane, which had no continuity with the Golgi cisternae or the endoplasmic reticulum; some inclusions had a clear association with small lysosomal granules. Immunofluorescence and immunoperoxidase studies using light microscopy failed to elucidate completely the nature of the inclusions, but immunocytochemical reactions performed using electron microscopy suggested an immunoglobulin nature. All inclusions were negative for acid phosphatase and periodic acid Schiff. The nature of the inclusions described in the prolymphocytes of this patient were compared with those previously recorded in B prolymphocytic leukaemia.