When intracytoplasmic sperm injection (ICSI) is performed in mice, isolation of sperm heads is usually performed prior to injections in order to increase the efficiency of the procedure. Consequently, the isolated sperm heads undergo an inevitable incubation in vitro. However, little is known about the effects of this incubation step on fertilization and embryo development following ICSI. When we incubated sperm heads at 37 A degrees C, there was a significant time-dependent decrease in fertilization and blastocyst formation. Moreover, the DNA integrity of the sperm heads was maintained over 12 h incubation. Using assisted oocyte activation, these defects in fertilization and embryo development were rescued. Taken together, incubation of sperm heads following isolation can affect the oocyte-activating capacity of sperm thereby compromising fertilization and embryo development associated with ICSI.