WE have already described the culture of cleaving human embryos to the sixteen celled stage¹, and we now wish to give details of a few embryos that have developed much further, including two that reached fully developed blastocysts. Methods were similar to those described before. Preovulatory oocytes recovered by laparoscopy² were fertilized in Bavister's medium³, and transferred after 12–15 h into Ham's F 10 supplemented with human or foetal calf serum. Preliminary details of this work have been presented elsewhere⁴.