B cell stimulatory factor (BSF) activity was generated over a period of five days by phytohemagglutinin-stimulated E rosette-positive peripheral blood lymphocytes. This activity was subjected to a multistep purification procedure including ammonium sulfate precipitation, anion-exchange chromatography, gel filtration, procion red-agarose chromatography and reverse phase high performance liquid chromatography (RP-HPLC). The last purification step dissected BSF activity into two active fractions, one corresponded to the interleukin 2 (IL 2) activity whereas the other active fraction was free of IL 2 activity. Preparative isoelectric focusing analysis defined isoelectric points of pH 7.2 for both BSF and IL 2. Molecular weight analysis of BSF was carried out by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. BSF activity was eluted from gel strips corresponding to a molecular mass of 16 and 17 kDa. The 16-kDa fraction was free of IL 2 activity whereas the 17-kDa fraction overlapped with IL 2. Since recombinant IL 2 was capable of exhibiting significant BSF activity in anti-IgM or Staphylococcus aureus Cowan strain I-dependent assay systems, it cannot be excluded that IL 2 itself is a BSF. Nevertheless these studies demonstrate the existence of a BSF free of IL 2 activity.