We report the cloning of bovine interleukin-6 (IL-6) cDNA. The clone was isolated from a bovine-leukemia virus (BLV)-induced B cell-lymphosarcoma cDNA library cloned in the bacteriophage lambda gt11. The cDNA encodes a full length IL-6 protein made of 208 amino acids with 65, 53, 42 and 42% homology to published sequences of porcine, human, mouse and rat IL-6, respectively. The significance of IL-6 expression in a BLV-induced tumor is briefly discussed.