TUMOURS consisting exclusively of plasma cells have been described with fair frequency as constituting one of the histological types of multiple myeloma. Thus Wallgren (1921) in a collected series of 125 cases found 35 which presented a typical plasma-cell structure, and during the last decade numerous other examples have been added to the literature. Solitary or localised plasma-cell tumours, on the other hand, are of much greater rarity and only a few instances have been recorded hitherto. They comprise both the solitary plasmocytomas of bone marrow, histologically identical with the multiple growths referred to but differing radically from them in extent and clinical behaviour, and the localised plasma-cell tumours which occur in the mouth and nasopharynx and more rarely in other mucous membranes. At Leeds we chanced to encounter four examples in little more than two years, and as they illustrate rather well certain of the peculiar features of these growths we venture to place them on record. Two were solitary plasma-cell myelomas of bone, arising in humerus and maxilla respectively, and two were of buccal origin, one of which occurred in the nasopharynx, the other in the floor of the mouth. All four were met with during the routine histological investigation of material removed a t operation.