A.Sato1, H.Yanagimachi2 and R.Yanagimachi2

3To whom correspondence should be addressed In a total of 1048 intracytoplasmic sperm injection (ICSI) cycles, motile spermatozoa from four out of 424 patients (0.9%) failed to fertilize oocytes, despite an apparently successful ICSI procedure. No activation was observed in these injected oocytes. The spermatozoa from three of the four patients were injected into unfertilized mouse oocytes by ICSI (mouse test) to evaluate their oocyte activating ability. The oocyte activation rate of the spermatozoa of patients A, B, and C in the mouse test was 46, 100, and 86 % respectively (control: 100%). Simultaneous injection of two spermatozoa from patient A into the mouse oocytes increased the oocyte activating rate to 89 % (sham control: 29%). 100 % fertilization rates were obtained for patients A and B by combining ICSI and electrical stimulation, and this resulted in pregnancy and the birth of healthy twins for the partner of patient A. Thus, it is considered that the spermatozoa of these patients are not lacking sperm factors but that the activity of these factors is depressed. The combination of ICSI and electrical stimulation is effective in these cases. Key words: electrical stimulation/infertility/intracytoplasmic sperm injection/oocyte activation/spermatozo