An IgM-secreting extramedullary plasmacytoma with macroglobulinemia presented itself in the skin. Reactions of the plasmacytoid cells in dermal nodules to erythrocyte-antibody-complement complexes suggested that the neoplastic cells differentiated in situ. Large numbers of plasmacytoid neoplastic cells co-mingled with alpha napthyl acetate esterase-positive histiocytes in a random and “territorial‘’ distribution, which suggested a functional relationship between them. The subject of extramedullary cutaneous plasmacytoma is reviewed and discussed.