Recent phenotypic analysis of plasma cells showed that normal plasma cells do express the B-cell lineage-specific molecule CD19, but their malignant counterpart (myeloma cells) are CD19-. To clarify the meaning of loss of CD19 antigen on myeloma cells, we first compared the expression of CD19 and Pax-5 genes among B cells, normal plasma cells, myeloma cell lines, and primary myeloma cells, because the Pax-5 gene was reported to encode the transcriptional factor, B-cell-specific activating protein (BSAP), necessary for CD19 gene expression. Neither CD19 nor Pax-5 mRNA could be detected in those primary myeloma cells and cell lines, whereas normal plasma cells did express both CD19 and Pax-5 mRNA. Furthermore, we could confirm that BSAP-binding activity was not detected in the nuclear extract from CD19- myeloma cell line (KMS-5) but was detected in CD19+ B-cell line (Raji) by gel-shift assay. We further examined the expression of E2A and Id genes, because E2A and Id are considered to be positive and negative regulators in the expression of Pax-5 gene, respectively. However, no significant differences in the expression of these E2A and Id-2 genes could be observed between myeloma cells and normal plasma cells. Therefore, these data suggest that the altered expression of Pax-5, but not E2A or Id, is responsible for the loss of CD19 expression in human myeloma cells, although the underlying mechanism of the altered Pax-5 gene expression remains to be clarified.