['To determine if production of the immunomodulatory protein, progesterone induced blocking factor (PIBF), requires merely progesterone or whether other factors made by the corpus luteum are required.', 'The percentage of peripheral lymphocytes expressing PIBF was determined by obtaining a blood sample from women 9-12 days after embryo transfer. The embryos were either fresh ones following hyperstimulation and oocyte retrieval or were frozen-thawed embryos. Preparation for frozen embryo transfer required corpus luteum suppression with exogenous estrogen. The percentage of lymphocytes expressing PIBF was determined by an immunocytochemistry method.', 'PIBF expression (>1%) was found in 20.5% of COH and 13.3% of frozen embryo transfer cycles. There either was a significant difference or a trend for higher pregnancy rates when PIBF expression was detected.'] Is a corpus luteum a prerequisite for the expression of progesterone induced blocking factor by T-lymphocytes a week after implantation?