The high rate of implantation failures in infertile patients after in vitro fertilization must be regarded as the major problem of the kind of treatment. Usually, no information on the development of the embryo can be obtained for the time between embryo replacement and rising beta-hCG levels. Own studies on the early pregnancy factor (EPF) showed a positive reaction few hours following the contact of a fertilized oocyte with the endometrial surface. Therefore, we used the EPF as a marker for the viability of the embryo in 82 patients after in vitro fertilization and embryo transfer. Within two days after embryo transfer the EPF was positive in 52 (63%) patients and negative in 30 (37%) patients. In these women the embryos may have been lost during handling or may have discontinued further development. Between day 3 and day 12 after transfer the EPF turned to negative values in 35 patients--especially between day 6 and 10. These cases must be regarded as true implantation failures. After day 12 following embryo transfer, rising beta-hCG levels could be measured in 17 women (21%), but only in 12 patients (15%) could a growing embryonic sac be detected by ultrasound. From these figures, we may conclude, that about half of the embryos are lost already during the step of embryo transfer and the other half during implantation. Therefore, more attention should be given to the handling of the embryos to increase the pregnancy rate after in vitro fertilization.