The aim of the present study was to determine the potential for the further developmentof zona-free mouse zygotes fertilized in vitro. Zona-pellucida of (C57BL/6J X C3H/HeN) F1 mouse eggs-were removed with 0.5% pronase 6 hr after insemination. Each one of zonafreeeggs were cultured for 114 hr in 20μ1 medium in Multiplate-Terasaki and 20μl and 100pl drops in dish. At 96 hr after insemination 58%, 74% and 66% of these eggs, respectively, developed into the blastocyst stage. After transfer to the uterus of foster-mothers, 4 (5%) of 76 blastocysts cultured in 20μl medium in Multiplate-Terasaki, developed into newborns.It was concluded that the zona-pellucida is not essential for development into the youngfrom the zygotes fertilized in vitro.