Ultrastructural examination of a biopsy of a myeloma kidney has shown structural relationships between the multinucleate giant cells which characterize this condition and macrophage polykarya. On the other hand, no evidence was found of any resemblance to renal tubular cells. Moreover the electron microscopic observations indicate that these multinucleate giant cells, in common with the mononuclear phagocytes, attempt to ingest the cast material which in addition becomes fibrillar in the zones of endocytosis. No such phenomena were observed in the vicinity of tubular cells. In conclusion the ultrastructural evidence suggests that the multinucleate giant cells of myeloma kidney are derived from mononuclear phagocytes.