Rat embryos were explanted at 9.5 days and cultured for 48 h in human serum supplemented with glucose. The embryos were anaemic and frequently malformed. The haemoglobin and DNA content of these embryos was less than those grown in pure rat serum. Addition of 10% rat serum improved the embryonic growth as well as the haemoglobin and DNA content. This suggests that rat serum provides essential growth factors required by the embryos which are not present in human serum.