Although sonication is a simple way to immobilize (“kill”) spermatozoa prior to injection into oocytes, this has been thought to be destructive to sperm chromosomes. Mouse and human spermatozoa were immobilized by sonication and kept in various media for up to 2 h, then their nuclei were individually injected into mouse oocytes for the analysis of chromosomes at the first cleavage metaphase. In both the mouse and human, incidence of structural chromosome aberrations was much higher in the spermatozoa sonicated and stored in Biggers-Whitten-Whittingham medium for 2 h at 37.5°C than in those stored for 5 min in the same medium. We concluded, therefore, that it is not sonication per se but a prolonged exposure of sperm nuclei to extracellular milieu that is detrimental to sperm chromosomes. The incidence of structural chromosome aberrations of mouse and human spermatozoa was significantly reduced when the spermatozoa were sonicated and stored in K -rich nucleus isolation medium containing EDTA. This suggests that sperm chromosome degradation following sperm immobilization by sonication is partly due to detrimental effects of a Na -rich medium and of DNase on sperm chromatin. Ideally, it should be possible to prepare artificial media that maintain the integrity of sperm chromosomes for many hours after immobilization.