Many transgenic mice expressing human growth hormone gene were infertile. To investigate the infertility of these transfenic mice, it was looked into the estrus cycle and sexual behaviour and also tested through in vitro fertilization whether the germ cells of these mice normal or not. The infertile female transgenic mice were mated to the fertile males of ICR strain, but in almost all of them the vaginal plugs were not detected and their estrus cycles by vaginal smear were almost irregular which kept up estrus or diestrus stage. Many male transgenic mice did not have the ability of sexual behaviour. Therefore the viability of germ cells in infertile male transgenic mice was investigated by in vitro fertilization, but the sperm were normally fertilized with the eggs and the transgene of parent was passed on to the progeny. These results consequently suggest that the infertility of transgenic mice experssing human growth hormone gene may be due to the physiological activity of human growth hormone, not germ cells.