The pro-inflammatory molecules, tumor necrosis factor α (TNFα), interleukin 1 (IL-1), interleukin 6 (IL-6), and prostaglandin E 2 (PGE 2 ), are postulated to have a role in human pregnancy and parturition. The ability of interleukin 4 (IL-4) to suppress the production of TNFα, IL-1, IL-6, and PGE 2 by activated monocytes prompted us to investigate a possible regulatory role for IL-4 in human gestation. Immunohistochemical techniques were used to show that human amnion epithelium stained positively for IL-4. Tissue from both the first (n = 5) and third (n = 46) trimester expressed immunoreactive IL-4, which was detected by the use of four antihuman IL-4 monoclonal antibodies. Analysis of mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR) on RNA extracts of amnion epithelial cells indicated that they were the source of IL-4. One of the anti-IL-4 antibodies used stained IL-4 protein associated with the basement membrane of the amnion epithelium. The mechanism of this association was investigated. IL-4 was shown to be a heparin-binding cytokine, which would enable it to bind to components of the extracellular matrix. Thus, this study identified a previously undescribed cellular source of IL-4, implicating a role for IL-4 in human gestation. Additionally, glycosaminoglycan binding may regulate IL-4 activity in vivo.