Disclosed is a composition comprising a substantially purified Cytokine Inhibitory Factor (CIF) having certain characteristics, including the ability to inhibit RNA transcription of the pro-inflammatory cytokines tumor necrosis factor alpha (TNF-.alpha.), interleukin-1-beta (IL-1.beta.), and interleukin-2 (IL-2). Also disclosed is a method for substantially purifying the Cytokine Inhibitory Factor and a method for modulating the immune system of an animal using such composition.