GM-CSF is a monomeric protein of 127 amino acids with two glycosylation sites. The protein is synthesized as a precursor of 144 amino acids, which included a hydrophobic secretory signal sequence at the aminoterminal end. The sugar moiety is not required for the full spectrum of biological activities. Non-glycosylated and glycosylated GM-CSF show the same activities in vitro. Fully glycosylated GM-CSF is biologically more active in vivo than the non- glycosylated protein. This protein is secreted together with other factors by T cells and macrophages following cell activation by antigens or mitogens. Approximately 90% of the secreted colony stimulating activities are due to GM-CSF.