The capacity of immunosurgically (IS) treated inner cell masses (ICMs) versus microsurgically (MS) isolated primitive ectoderms from blastocysts recovered on the 5th day of gestation to regenerate an external layer of endoderm cells in vitro was investigated. While the majority of IS-treated ICMs regenerated such a layer, MS-isolated ectoderms seldom did so. Examination of the two types of tissue fragments revealed that IS-treated ICMs almost invariably retained viable endoderm cells whereas MS-isolated ectoderms did so only exceptionally. The endoderm was found to be more than one cell layer thick in ICMs from 5th day blastocysts, suggesting that some endoderm cells survive IS because they are protected from exposure to antiserum. Typing of the endoderm layer that regenerated following IS treatment of recombinant ICMs composed of genetically dissimilar endoderm and ectoderm provided direct evidence that it originated from residual endoderm cells rather than the underlying ectoderm. Finally, blastocyst injection experiments confirmed that IS-treated ICMs behave like a mixture of ectoderm and endoderm tissue in vivo, and provided no support for the view that cells of the original and regenerated endoderm differ in developmental potential. These findings challenge earlier conclusions concerning cell lineage and determination in the primitive ectoderm that were based on development in vitro of IS-treated ICMs from giant blastocysts.