To evaluate the diagnostic value of Early Pregnancy Factor (EPF) measurements in infertile patients, we performed a modified rosette inhibition test in 58 women after follicular stimulation and/or homologous artificial insemination (AIH). The patients were grouped according to the main diagnosis as cervical hostility, poor semen quality, and anovulation. While the overall fertilization rate in our patients was 39.6%, which is considerably lower than in comparable studies of fertile women, the pregnancy rates per EPF positive cycle in the various groups were 25.0%, 30.8%, and 50.5%, respectively. As the process of fertilization can be detected by EPF measurement within 48 h after ovulation, a distinction can be made between failure of fertilization and of implantation. Thus, a valuable diagnostic tool has become available for treatment of infertile patients.