In multiple myeloma (MM), the cell surface protein, CD19, is specifically lost while it continues to be expressed on normal plasma cells. To examine the biological significance of loss of CD19 in human myeloma, we have generated CD19 transfectants of a tumorigenic human myeloma cell line (KMS-5). The CD19 transfectants showed slower growth rate in vitro than that of control transfectants. They also showed a lower capability for colony formation as evaluated by anchorage-independent growth in soft agar assay. The CD19 transfectants also had reduced tumorigenicity in vivo when subcutaneously implanted into severe combined immunodeficiency (SCID)-human interleukin-6 (hIL-6) transgenic mice. The growth-inhibitory effect was CD19-specific and probably due to CD19 signaling because this effect was not observed in cells transfected with a truncated form of CD19 that lacks the cytoplasmic signaling domain. The in vitro growth-inhibitory effect was confirmed in a nontumorigenic human myeloma cell line (U-266). However, introduction of the CD19 gene into a human erythroleukemia cell line (K-562) also induced growth inhibition, suggesting that this effect is CD19-specific, but not restricted to myeloma cells. These data suggest that the specific and generalized loss of CD19 in human myeloma cells could be an important factor contributing to the proliferation of the malignant plasma cell clones in this disease.