SUMMARY: The objective of this study was to test whether the developmental capacity of human multi-pronuclear (PN) zygotes after ultra-rapid freezing using EM grid can be maintained. For this experiment, multi-PN zygotes which produced in human IVF program were used as an altematives of normal 2PN zygotes, and they were separated into 3PN or > or =4PN zygotes to compare their in vitro development and cryoinjury according to PN number. As freezing solution. EFS30 which consisted of 30% ethylene glycol, 18% ficoll, 0.5 M sucrose and 10% FBS added D-PBS was used. The result obtained in this experiment was summarized as follows; When the multi-PN zygotes were ultrarapidly frozen and thawed, the high mean percentages (85.5%) were survived. Also when the cleavage rates between control and freezing group were compared with PN number, there were not significantly different in each group (3PN; 81.3% & 85.4% and > or =4PN; 90.0% & 95.7%). When the in vitro development rates after thawing were examined, freezing 3PN group (22.0%) was not differed to control 3PN group (38.5%), although the development result of freezing > or =4PN group (4.5%) was significantly lower than that of control > or =4PN group (44.4%) (p<0.05). These results demonstrate that developmental capacity of human zygote can be obtained by ultra-rapid freezing method using EM grid and EFS30.