Eighty-five human embryos fertilized in vitro were frozen and thawed with 1,2 propanediol as a cryoprotective agent. The effects of viability of in vitro culture duration, stage, and morphologic appearance of embryos were examined after thawing. Survival was higher for 2-day-old embryos than for 3-day-old embryos (56% versus 18%) and for 2-, 4-, and 8-cell embryos than for intermediate-cleavage-stage embryos (67% versus 22%). Among 19 regular-cell-size embryos at the 2-, 4-, and 8-cell stage, 15 (79%) kept 50% or more of their initial number of blastomeres after thawing and 9 were intact. The average viability of all 2-day-old frozen-thawed embryos can be estimated at 19%.