A cytotoxic factor was extracted from the spleen of 2 individuals with chronic myelocytic leukemia (CML), of 2 with primary hypersplenism, and of 4 normal persons who underwent splenectomy because of trauma. Its cytotoxic activity was tested on normal human fibroblast and CML and multiple myeloma (MM) cell lines of human origin developed at our laboratory. The factor was cytotoxic for fibroblasts, CML, and MM cells in cultures, inhibited RNA synthesis, and had a secondary effect on DNA synthesis and mitotic activity. The active factor had a molecular weight of about 1,000 daltons and was probably a peptide or glycopeptide.