A myeloma cell line, MM.1, has been established from the peripheral blood cells of a patient with immunoglobulin A myeloma. MM.1 grows in suspension either singly or in small clusters and secretes lambda-light chain. Phenotypically, MM.1 cells lack most B cell antigens, but they do express human leukocyte antigen DR, PCA-1, and T9 and T10 antigens. Molecular analysis of MM.1 demonstrates that it is negative for the presence of the Epstein-Barr virus genome. Southern analysis of MM.1 detected a rearrangement of the lambda-light chain gene, and Northern analysis revealed high levels of lambda gene expression. Cytogenetic analysis of the MM.1 cell line revealed the presence of seven related chromosomally abnormal cell lines characterized by numerical and structural aberrations, and it revealed five nonclonal abnormal cells. The most notable abnormality is a reciprocal translocation involving band q24.3 of chromosome 12 and band q32.3 of chromosome 14; translocations involving 14q32 are frequently observed in neoplasms of B cell origin.