Mouse blastocysts were microsurgically injected with embryonal carcinoma cells from in vitro teratocarcinoma cell lines C17, C86, SIKR-OSB, and PCC3/A/I. The embryos were allowed to develop to term and the resulting offspring were analysed for chimaerism using coat colour markers and isozyme differences of the enzyme glucose phosphate isomerase. When injected into blastocysts, cell line C86 produced tumours in six of 74 animals born. The tumours were detected at birth and were poorly differentiated neuroectodermal teratocarcinomas. Cell line C17 gave 13 chimaeras in 77 mice born, five of which showed chimaerism only in normal tissues, mainly melanocytes of the coat and eye. The other eight chimaeras developed tumours. Seven of these developed in adult animals and were mainly fibrosarcomas. Cell line SIKR-OSB resulted in one normal chimaera in 44 mice born. Of 86 animals born following injection of cell line PCC3/A/1, there was one chimaera with a small tumour and three normal chimaeras. The levels of chimaerism were generally very low. The mice were test bred but with no evidence of germ line chimaerism. The karyotypes of all the cell lines were abnormal. How this and other factors such as cell cycle times might affect the incorporation of embryonal cells into the developing embryo is discussed.