The National Council on Medical Ethics has sent a letter to the Government to direct its attention to developments in biological research with regard to the mixing of human and animal material. The letter describes shortly the historical and current use of animal-human mixes in research. It then focuses on recent developments in stem cell research where the lack of human ova has led to trials in certain countries where human somatic cell nuclei are introduced into rabbit or cow ova to produce embryonic stem cells. The Council refers to a report by one of its members, Professor E. Rynning, to the European research network Chimbrids, where she concludes that Swedish regulations are not sufficiently up-to-date to deal with these matters. The Council requests that the Government addresses ethical and legal aspects of research with human-animal mixtures.