Abstract: The present invention relates to the field of diagnosis and therapy of hematological malignancies, such as multiple myeloma, as well as solid tumors based on cytokine interleukin-16 (IL-16) and agents specifically targeting this antigen or cells expressing the same, e.g., antibodies. The inventors were able to prove that IL-16 is expressed and secreted at high levels by myeloma cells. Most importantly, the inventors have demonstrated for the first time that IL-16 supports the proliferation of the malignant cells. Therefore, this cytokine represents a particularly advantageous target in cancer therapy and diagnosis.