Freezing of embryos deprived of the zona pellucida (ZP) decreases their survival rate immediately after thawing, and gives rise to the separation of their blastomeres in a high percentage of cases. We have studied the ultrastructure and the characteristics of actin fibers in the cell-to-cell contact region in mouse embryos frozen-thawed without the ZP at the two-cell stage. Our results indicate that most of the embryos that retain their blastomeres united after freezing and thawing show either the presence of a midbody, or a contact region with a close apposition of the plasma membranes but without an organized actin cortex in their contact region. Only a small percentage of embryos that retain their blastomeres united after freezing and thawing show a contact region with widely separated plasma membranes and an organized actin cytocortex. © 1997 Academic Press.