Murine Interleukin 2 (IL2) was denatured with sodium dodecyl sulfate (SDS) with or without concomitant reduction of disulfide bonds. Between 50 and 100% of the activity was recovered upon removal of SDS. When SDS-denatured IL2 was chromatographed on a calibrated gel filtration column in the presence of SDS, it eluted with proteins of m.w. 16,000. This value is supported by sedimentation velocity studies in SDS-containing glycerol gradients. Three activities previously associated with IL2, namely the obligatory role in thymocyte mitogenesis, helper activity in the generation of cytotoxic T lymphocytes, and T cell growth factor activity, co-purified after SDS denaturation. These results indicate that the essential component of murine IL2 is a peptide of m.w. about 16,000. The 3 species of Interleukin 2 studied so far--rat, human, and murine--thus can all exist as polypeptide chains of 15,000 to 16,000 m.w. The murine factor is normally isolated as a larger entity, of about twice this m.w.