Objective: To develop an effective ICR mouse embryo culture medium. Design: In vitro model study. Setting: University-affiliated hospital. Animals: Four-week-old, superovulated mice. Intervention(s): In vivo– or in vitro–derived one-cell embryos were cultured in preimplantation-1 medium (P-1). Main Outcome Measure(s): Preimplantation development. Result(s): In vivo-derived embryos were cultured in BSA-containing P-1, to which one of the following substances was added: [1] no addition, [2] amino acids (aa), [3] aa+hemoglobin (hb), [4] aa+hb+cysteine (cys), [5] aa+hb and glucose (glu) added at the four-cell, or [6] aa+hb and glu+cys added at the four-cell stage. More ( P<0.05) blastocysts developed after aa or aa+hb addition than after no addition, and glu addition to such medium further stimulated the formation (54%). In P-1 with aa+glu, the addition of 1 μg/mL hb was optimal. Additional improvement of blastocyst formation (78%) was achieved by ethylenediaminetetraacetic acid (EDTA), supplementation and bovine serum albumin replacement with polyvinyl alcohol (PVA) did not inhibit the development. P-1 supplemented with aa, hb, glu, EDTA, and PVA also supported the development of in vitro–derived embryos (70%). Conclusion(s): A modified P-1 medium was developed, and it supported the development of both in vivo- and in vitro-derived ICR mouse embryos.