Abstract Tunicamycin, an antimetabolite which inhibits the N-glycosylation of proteins, does not block the initial cleavages of mouse embryos, even at relatively high concentrations. However, it can interfere with compaction and blastocyst formation. Although tunicamycin treatment from the two-cell or eight-cell stage can cause developmental arrest prior to hatching from the zona pellucida, much higher (sublethal) concentrations of the antimetabolite added at the morula or blastocyst stage do not specifically affect hatching of blastocysts, their attachment to the substratum, or outgrowth of trophoblast cells. The consequence of continuous exposure of embryos to moderate amounts (0.05 to 0.1 μg/ml) of tunicamycin through peri-implantation stages is death of trophoblast cells with little effect upon the cells of the inner cell mass (ICM). The latter give rise to apparently normal early endoderm cells in the presence of the antimetabolite. The incorporation of leucine, mannose, and fucose into acid-insoluble material by ICM cells is only minimally inhibited by tunicamycin. On the other hand, the antimetabolite causes a severe inhibition of incorporation of not only mannose, but also leucine, into acid-insoluble material in trophoblast cells. Thus, trophoblast cells resemble transformed cells by their extreme sensitivity to tunicamycin.