AbstractA search for an endogenous cytotoxic factor (ECF) was made by analyzing 103 spleens from normal humans and patients suffering from hematopoietic malignancies, solid tumors, inflammatory diseases, and congestive and hyperplastic splenomegalies. A highly purified ECF was obtained by a combination of gel filtration and ion‐exchange column chromatography. The factor is a low‐molecular‐weight (<2,000) substance and is probably a peptide or peptide‐nucleotide complex. The biologic activity of the ECF was assayed on myelogenous and lymphatic leukemia cells of human origin. The spleens from normal individuals produced and/or stored the largest quantity of the ECF. The amount of ECF extractable from the pathologic spleen was drastically diminished regardless of the disease or therapeutic regimen. The ECF was significantly more cytotoxic for lymphatic than for myelogenous leukemi