Recent reports indicate that SP1, a "pregnancy-specific beta 1-glycoprotein", can be used as a biological marker for very early pregnancy and occult abortion. In this investigation, SP1 serum concentrations were measured in the luteal phase of 48 menstrual cycles stimulated for in-vitro fertilization (IVF) and embryo transfer (ET). All patients received hMG for ovarian stimulation. Ovulation was induced by beta-hCG and also administered to support the luteal phase. In the 8 pregnancies arising after ET, SP1 (less than 0.5 ng/ml) was not detected before 13 to 19 days after laparoscopy. In contrast, the pregnancy-dependent beta-hCG increase was detectable earlier than SP1 despite the administration of hCG given for luteal support. However, low SP1 readings (0.5-1.1 microgram/ml) as early as 3 days after laparoscopy were observed in 11 cycles without a positive sign of beta-hCG production. Our results suggest that SP1 determinations cannot be used as a marker for occult abortion; also, positive SP1 readings without increase beta-hCG, especially during the early luteal phase after ET, have to be interpreted with caution.