This study was carried out to investigate the inhibiting or promoting effect of fetal bovine serum fractionated by the molecular weight and to examine the effect of reconstruction of serum fractions on the development of 1- and 2-cell mouse embryos fertilized in vitro (IVEE) . The serum was separated by ultrafiltration or gel filtration methods and added in m-KRB medium for culture of IVFE. The developemental ability(cavitation and hatching) of embryos following culture of day 4 and 6 was compared among fractions. Small molecular weight fraction( <3 kDa) significantly inhibited the development of 1-and 2-cell IVFE to the blastocyst stages, compared with other fractions. One-cell IVFE were more sensitively damaged than 2-cell embryos by that fraction and arrested mainly at 2~4 cell stages. Moreover, small amount(<3%,v /v) of the inhibiting fraction acted even with protein rich fraction(100~30 kDa) and arrested the embryonic development. On the other hand, 100~30 kDa fraction promoted the embryonic development and no inhibiting effect was observed at the level of 50%(v /v) in culture medium In the experiment of gel filtraton, =30 kDa fraction showed the highest promoting effect on the embryonic development, but <4 kDa fraction inhibited significantly the development. These results suggest that serum contains not only small molecular weight inhibitory component(s) but also promoting one rather than albumin on embryonic development. And serum can be more effectively used in the IVF program after removal of inhibitory component(s) by one of above separation methods.