1. This limited study has revealed plasmacytosis of 5.4 per cent to 23.6 per cent in the bone marrow of six diseases other than the primary plasmacytic diseases.
2. The plasmacytic elements observed in these responses were predominantly mature varieties consisting of plasmacytes and degenerative plasmacytes.
3. In each of the cases described a coexisting increase of reticulum cells was observed in the bone marrow.
4. Plasmacytic elements with nuclear and cytoplasmic characteristics of reticulum cells were described and lend additional morphologic evidence to the suggested reticulum cell origin of the plasmacytic series.
5. Even though a morphologic analysis of these cells has revealed a predominance of plasmacytes and degenerative plasmacytes it has not been suggested that plasmacytic responses can be distinguished from the primary plasmacytic diseases on the morphologic appearance of the plasmacytic elements in the bone marrow. Rather, the demonstration of bone marrow plasmacytosis should be subjected to careful evaluation with consideration of other diagnostic criteria before a diagnosis of primary plasmacytic disease is made.