Direct injection of a single spermatozoon into an oocyte (ICSI) can produce apparently normal offspring. Although the production of normal offspring by ICSI has been successful in mice and humans, it has been less successful in many other species. The reason for this is not clear, but could be, in part, due to inconsistent activation of oocytes because of delayed disintegration of sperm plasma membrane within oocytes and incorporation of the acrosome containing a spectrum of hydrolyzing enzymes. In the mouse, the removal of sperm plasma membrane and acrosome was not a prerequisite to produce offspring by ICSI, but it resulted in earlier onset of oocyte activation and better embryonic development. The best result was obtained when spermatozoa were demembranated individually immediately before ICSI by using lysolecithin, a hydrolysis product of membrane phospholipids.mouse ͉ human ͉ Ca 2ϩ oscillations ͉ fertilization ͉ lysolecithin D irect injection of a single spermatozoon into an oocyte, commonly called intracytoplasmic sperm injection (ICSI), can produce apparently normal offspring even though it bypasses a number of biological processes necessary for normal fertilization. As long as the sperm nucleus has intact genetic integrity, ICSI can produce healthy offspring regardless of concentrations, morphology, and motility of spermatozoa (1, 2). Only one genomically normal spermatozoon is needed to fertilize one oocyte. A salient difference between natural and ICSI fertilization is that, in the latter, the sperm plasma membrane as well as acrosome (which contains a spectrum of powerful hydrolyzing enzymes) are introduced into an oocyte. For species with small acrosomes, injection of the acrosome into an oocyte apparently does not produce serious problems, but for species like the hamster, with very large acrosomes, injection inevitably results in death of the oocyte (3). We had demonstrated that the contents of the acrosome are potentially harmful to oocytes (4). A notable difference between normal and ICSI fertilization is that repetitive transient increases in intracellular Ca 2ϩ concentration of the oocyte (Ca 2ϩ oscillations), the pivotal signal for oocyte activation (5-8), begins much more slowly in ICSI oocytes than in normally fertilized oocytes. In the mouse, for instance, Ca 2ϩ oscillations begin 1-3 min after plasma membrane fusion between a fertilizing spermatozoon and an oocyte (9), whereas oscillation begins 15-30 min after ICSI (10, 11). In human oocytes, Ca 2ϩ oscillations start 4-12 h after ICSI, when a plasma membrane-intact spermatozoon is injected after immobilization by touching the terminal part of the tail (12). Ca 2ϩ oscillations begin faster (14 Ϯ 6 min) when a spermatozoon is immobilized by applying several piezo pulses to the proximal one-third of the sperm tail before injection (13). Kasai et al. (14) reported that oocyte activation, assessed by the completion of meiosis, occurred earlier when spermatozoa were freed from the plasma membrane before ICSI. Increased fertilization rates aft...