Pluripotent cell development in the mammalian embryo results in the sequential formation of several developmentally distinct populations, inner cell mass, primitive ectoderm, and the primordial germ lineage. Factors within medium conditioned by HepG2 cells (MEDII) have been implicated in the formation and maintenance of primitive ectoderm from inner cell mass cells both in vitro and in vivo. Here we demonstrate that MEDII, but not LIF, is able to support the maintenance and proliferation in culture of pluripotent cells derived from primitive ectoderm formed in vitro or during embryonic development. This distinguishes primitive ectoderm and inner cell mass (ICM) on the basis of cytokine responsiveness and validates the biological activity proposed for factors within MEDII in primitive ectoderm establishment and maintenance. Further, it potentially provides an alternative technology for the isolation of pluripotent cells from the mammalian embryo.