We have recently described a heat‐labile and cell‐directed neutrophil migration inhibitory activity that is present in serum from patients with chronic lymphocytic leukaemia (CLL). The inhibitory activity is produced and secreted by CLL cells in vitro. In the present study the inhibitory activity was partially purified from short‐term cultures of monoclonal leukaemic B‐CLL cells. on gel filtration the calculated molecular weight was apparently 30,000. By anion exchange chromatography, the inhibitory factor was recovered in the fractions that eluted with 0.3 mol/l NaCl. The active material applied to preparative agarose gel electrophoresis migrated towards the anode. The inhibitory factor was totally destroyed by trypsin. In addition it bound to concanavalin A‐Sepharose. These properties indicate that the inhibitory factor is a glycoprotein. Antibodies against the isolated inhibitory factor were raised in rabbits. CLL serum was separated by means of gel filtration and the inhibitory activity was recovered in the pool with a molecular weight of approximately 30,000. The activity was completely neutralized by the antibodies raised against the CLL‐cell‐derived inhibitor, indicating the similarity between this and the serum‐derived inhibitor. We have shown the existence of a new lymphokine, derived from B‐CLL cells, in serum from