The distribution of immunoglobulins and secretory component (SC) in the human endometrium has been studied by an immunoperoxidase technique. SC is present in a proportion of the epithelial cells and the amount of this substance is increased during the secretory phase of the cycle. A variety of immunoglobulins are present, in low concentrations, in the stromal interstitium during the secretory phase of the cycle and these are thought to diffuse passively from the plasma as a non-specific accompaniment of stomal oedema. Only IgA is found in the epithelial cells and this appears solely in those cells containing SC. No immunoglobuln-containing lymphoid cells are present in the endometrium. It is suggested that the endometrium lacks a true local secretory immune system but is able, because of its content of SC, to compensate for this by extracting polymeric IgA from the plasma. This system presumably helps to protect the endometrium against infection but the biological significance of its apparent control by progesterone is uncertain.