IN RECENT YEARS an increased percentage of plasma cells in the bone marrow and other tissues of the body has been noted in many diseases other than the primary malignant plasmacytic diseases (multiple myeloma, diffuse plasma cell myelosis, and plasma cell leukemia). This condition has been termed plasmacytosis. Plasmacytosis may be considered of clinical importance for two reasons: 1. It requires differentiation from malignant plasmacytic disease. 2. Its diagnostic and clinical significance requires appraisal. In this study we wish to review our experiences with 50 cases of plasmacytosis of the bone marrow during the past six years. MATERIAL AND METHODS Between Jan. 1, 1948, and June 15, 1953, a total of 727 bone marrow examinations were performed at Parkland Hospital. Of these there were 76 preparations which showed a plasma cell count of 5% or greater. Five per cent has been arbitrarily selected as the level at or above which