['To compare the expression by T-lymphocytes of an immunomodulatory protein known as progesterone-induced blocking factor (PIBF) in conception versus non-conception cycles even when there has been definite fertilization and embryo formation.', 'PIBF expression on T lymphocytes was measured using an immunohistochemical method with a PIBF-specific polyclonal antibody. These levels were determined in patients undergoing three types of therapy: non-in vitro fertilization (IVF), IVF-embryo transfer (ET), and frozen ET. Sera were drawn 12 days from ovulation in non-IVF cycles or 9 days after ET and were assayed for PIBF and beta human chorionic gondotropin. Comparison of the frequency of lymphocyte expression of PIBF in pregnant versus non-pregnant women were made.', 'PIBF was detected in 29.5% of non-pregnant women and 52.5% of pregnant women. There were no differences in PIBF levels by therapy used in non-pregnant cases or in the pregnant group.'] Is evidence that the expression of progesterone-induced blocking factor by maternal T-lymphocytes positively correlated with conception?