Apparently vitrified cells and tissues often fail to survive, probably from damage from growth of microscopically invisible ice crystals. Special biological antifreezes from some polar fishes have been shown to adsorb to specific faces of ice crystals and inhibit crystal growth. Vitrification in the presence of antifreezes therefore may help enhance postvitrification viability of cells and tissues. We report here that the addition of fish antifreeze glycopeptides (AFGPs) to vitrifying solutions increases post-thaw viability in cultured immature pig oocytes and two-cell stage embryos of mice and pigs after rapid cooling to cryogenic temperatures. The criterion for viability is maturation to metaphase for the oocytes and the ability to develop into the four-cell stage for the pig embryo and the blastocyst stage for the mouse embryo. Without AFGPs, or with addition of antifreeze peptides (AFPs), the particular vitrifying solution and cooling/warming/culturing regime used in this study produced zero viability. In the presence of the AFGPs (40 mg/ml), survival of pig oocytes and embryos was increased to about 25%, and that of mouse embryos to 82%. Dose-response studies for the mouse embryos showed that the protective effect of AFGPs shows saturation kinetics and levels off at 20 mg/ml. The AFGPs appeared to preserve cell membrane structural integrity; however, an intact cell membrane did not always lead to viability. The absence of protective effect by AFPs suggests that protection by the AFGPs is unrelated to their common antifreeze property, i.e., inhibition of ice crystal growth, but probably results from interaction with and stabilization of the cell membranes unique to the AFGPs.