Recombinant IL4 is a bioactive protein intended for use in cell culture applications. IL4 promotes the proliferation and differentiation of activated B cells. In addition, IL4 up-regulates class II MHC antigen expression and IgE receptors. ED50 = 0.05 to 0.4 ng⁄mL (Specific Activity: 2.0 x 10^7 to 2.5 x 10^6 units⁄mg). The biological activity is determined by measuring the dose-dependent proliferation of human TF-1 cells. A concentration range of 0.1 to 10.0 ng⁄mL is effective for most in vitro applications.