In early life humans derive all their nutrients from milk. Human milk provides the neonate with not only a food source but also a source of biologically active molecules such as hormones, growth factors and cytokines. Cytokines are multifunctional glycoproteins involved in cell communication and immune system activation. Two of these cytokines, granulocyte-colony stimulating factor (G-CSF) and interleukin-6 (IL-6), are present in variable concentrations in human milk. We measured G-CSF and IL-6 concentrations in healthy nursing mothers. The 30 milk samples assayed contained G-CSF levels from 14 pg/mL to > 2500 pg/mL. Of the 28 milk samples assayed for IL-6, 24 contained levels from 0.81 pg/mL to 306 pg/ mL. The remainder had levels below the detection limit of the assay. These biologically active molecules may play a role in neonatal growth and development, and may also protect the infant against infection.