BSF-1 was partially purified from serum-free culture supernatants of cells of the EL-4 thymoma line, which had been induced 48 hr earlier with 4 beta-phorbol-12 beta-myristate-12 alpha-acetate (PMA). BSF-1 in 10-liter batches was adsorbed onto and eluted from trimethylsilyl-controlled pore glass beads (TMS-CpG) and then subjected to reverse-phase high-performance liquid chromatography (RP-HPLC). The recovery of BSF-1 activity by TMS-CpG and RP-HPLC ranged from 52 to 55% and 187 to 227%, respectively. The specific activity in units per milligram of protein of partially purified BSF-1 was approximately 2600 times higher than that of the culture supernatant protein. The partially purified BSF-1 had a single isoelectric point of 6.3 and an apparent m.w. between 18,000 and 21,700 when analyzed by isoelectric focusing and gel filtration-HPLC, respectively. The ability to prepare large amounts of partially purified BSF-1 by a rapid and efficient procedure should be of great help in both biochemical and immunologic studies of this lymphokine.