Multiple myeloma. Light micrograph of a group of plasmoblasts (precursor plasma blood cells, purple) from bone marrow in a case of multiple myeloma, also known as Kahler's disease or plasma cell myeloma. Myeloma is a cancer of plasma cells, a type of white blood cell. In this condition, large numbers of plasma cells are present, normally only a minor constituent of normal bone marrow. The other cells (orange) are red blood cells.