A human plasmacytoma cell line (AMO1) was established. The AMO1 cells had the light and electron microscopic characteristics typical of plasmacytoma cells and did not harbor Epstein-Barr virus. These cells expressed cytoplasmic immunoglobulin A kappa and the immunoglobulin heavy-chain gene (JH) and kappa light-chain gene (C kappa) were rearranged. Coexpression of a CD4 antigen and plasma cell antigens (CD38 and PCA-1) was an unusual and sustained feature. Neither the T-cell receptor beta nor the gamma chain gene displayed the rearranged form. Other lineage-specific surface antigens, namely T, B, monocytoid, and myeloid antigens, were all negative in AMO1. In accordance with the surface CD4 expression, polymerase chain reaction analysis indicated constitutive expression of CD4 mRNA, and the cytogenetic findings revealed that AMO1 cells had a derivative chromosome 12, which had a structural abnormality of the short arm carrying the CD4 gene locus. These findings provide strong evidence for the presence of CD4-positive malignant plasma cells and raise the possibility that the CD4 expression in the AMO1 cell line is closely associated with the derivative chromosome.