Using a serum-free defined medium, we have established a human cell line, NCI-H929, from a malignant effusion occurring in a patient with IgAk myeloma. The cultured cells have the morphologic, ultrastructural, biochemical, immunologic, and cytochemical features of plasma cells. The cells have rearranged alpha and kappa genes and synthesize and secrete high amounts of IgAk (greater than 80 micrograms/10(6) cells per 24 hours). The cells express surface immunoglobulin (alpha and kappa), the plasma cell antigen PCA-1, the transferrin receptor (T9) and T10 but lack antigens associated with earlier stages of B cell development (HLA-DR, B1, B2, B4, CALLA), as well as other leukocyte- macrophage antigens and Epstein-Barr virus (EBV) nuclear antigen. Although molecular studies confirm that both the tumor and cultured cells are derived from the same clone of malignant B cells, the tumor cells were predominantly near-diploid, whereas the cultured cells are predominantly near-tetraploid with six copies of chromosome 8, four to six of which have an 8q + abnormality. However, both the tumor and the cultured cells have a rearrangement of the cellular c-myc proto- oncogene (located at 8q24) and express c-myc RNA. Although a modest number of human “plasmacytoid” cell lines have been established, most are lymphoblastoid lines lacking plasma cell features, while others appear to be early secretory cells. In contrast, NCI-H929 is a differentiated, highly secretory human plasma cell line.