In a cross-sectional study, the serum levels of pregnancy-specific beta 1-glycoprotein (PSBG), hCG, human LH, and progesterone were measured by RIAs during 94 mid or late luteal phases of 69 women using oral contraceptives. Subsequent spontaneous menstruation took place in every cycle. None of the women using oral contraceptives had any PSBG or hCG-like immunoreactivity in serum. In women with intrauterine devices, PSBG was found in six cycles (6.4%) and hCG-like immunoreactivity was demonstrated in one cycle only, where PSBG also was present. In two out of six PSBG-positive cycles, menstruation was delayed by 5 and 16 days. Although rare, the transient occurrence of trophoblastic antigens in women using intrauterine contraception is taken as evidence for an occult pregnancy in these apparently infertile cycles.