A human cell line (LB 84–1) has been established from the bone marrow of a patient with Bence-Jones myeloma. Coexpression of plasma cell (Leu[CD38]) and myelomonocytic antigens (Leu MI[CD15], Leu M5 [CD11c], MY7 [CD13] plus butyrate and chloracetate esterase) proved to be an unusual but sustained feature of this cell line. The plasma cell phenotype with multinuclearity was retained. Shared major chromosomal abnormalities (del  [p14], t[5;?] [q35;?], del  [q21], and del 7[q32]) between the direct and cell line karyotypes affirmed the LB 84- 1 cell as being derived from the original patient myeloma clone. The mechanisms potentially responsible for the aberrant coexpressed phenotype are discussed. This myelomonocytic myeloma cell line will hopefully prove to be a valuable tool for the study of the genotypic and phenotypic evolution of human myeloma.