Interleukin 11 (IL-11) is a multifunctional cytokine that has diverse effects on blood cells and their precursors and on a number of cell types outside of the hematopoietic system. The cDNAs encoding murine IL-11 and its receptor alpha-chain (IL-11R alpha) have recently been isolated. We have used the RNase protection assay to examine the expression of murine IL-11 and IL-11R alpha in a range of adult mouse tissues, in embryos, and during development of embryonic stem (ES) cells into cystic embryoid bodies in vitro. The testis showed a high level of IL-11 gene expression while a much lower level of expression was detected in the lung, stomach, small intestine, and large intestine. Expression of IL-11 was not detected between day 10.5 and day 18.5 post coitum of embryonic development or in differentiating ES cells in vitro. In contrast, the IL-11R alpha was found to be expressed in all adult tissues examined, during embryonic development, and in totipotent and differentiating ES cells.