This research was conducted to obtain the basic information on the cell block phenomenon occuring during early development in vitro of mouse embryos. Early embryos were recovered at 3h post-hGG injection(hph). Various chemicals (EDTA, EGTA, DTPA, MA and PRA) were tested to examine the effects of them on the overcoming the 2-cell block phenomenon. One hundreds $mu$M of the chelating agents were added to the M16 medium containing embryos. The treated embryos were worked and transferd to fresh M16 medium after 1, 3, 6 and 12h of treatment. Development was examined at 58 and l2Oph injection, respectively. 44.7~68.9% of the treated embryos developed to 4-cell stages at 58hph. Only 17.6~60.3% of the embyos developed upto blastocyst at l20hph. Whereas control embryos showed slightly lower development in M16 medium alone (38.9~42.4%, 4-cell and 3.8~65.5%, blastocyst). Three mitogenic agents were tested. 51.6~63.8% and 43.4~48.1% of embryos developed up to 2-cell and blastocyst stage, respectively when treated in 5$mu$ g PHA-M Imi for 5 min, 1, 3 and 6h subsequently cultured in fresh M16 medium. Control embryos only showed 38.8% for 4cell and 5.9% fo blastocyst at 58 and l2Ohph, respectively. 100$mu$M PMA was also beneficial for the 2-cell block. Showing better development them that of control (42.4 vs 57.9~59.4% 4cell and 5.9 vs 25.0~55.6% blastocyst, respectively. However 1$mu$M butyric acid was toxic to early embryos, thus arresting further development. These result indicate that either chelating or mitogenic agents could be used to overcome the "in vitro 2-cell block" occuring during early development in vitro of ICR embryosCR embryos