To learn why some zygotes remain viable after gene transfer while others lyse, we injected DNA into fertilized eggs and compared those that lysed within 1 h of injection with others that retained a normal appearance. Using scanning electron microscopy, we found open holes on the surface of lysed eggs, indicating failure of the plasma membrane to reseal after microinjection. No holes were seen in unlysed eggs, but many of them had membrane alterations suggestive of healed punctures. We also examined aspects of the gene transfer procedure that might influence survival such as the size of injection pipettes and their taper relative to zygote diameter, possible toxicity of the injection medium, the timing of injection, and immediate vs. delayed pipette withdrawal. The only factors that significantly affected cell viability were pipette size and taper, and timing of injection in relation to first cleavage. This suggests that zygote viability correlates inversely with the size of the hole produced by the injection pipette and that damage to the membrane is less successfully repaired as the fertilized egg readies itself for division.