In a prospective study, the correlation between serum levels of placental isoferritin (PLF) and outcome of pregnancy was determined in 56 pregnant women. Women with contractions before the 36th week of pregnancy showed significantly lower PLF values compared with women with later contractions (p < 0.01). Furthermore, a strong correlation of PLF levels with birth weight was observed. In 11 (79%) cases with a birth weight < 2,500 g (group A), PLF values were 2,500 g (group B) revealed PLF levels < 10 U/ml. Because it has been shown previously that PLF has immunosuppressive properties, the secretion of PLF by the placenta could be responsible for the inhibition of the immunoreactivity of the maternal lymphocytes against the embryo. The strong correlation of low PLF values with preterm contractions and/or low birth weight recommends the determination of this protein as a marker for monitoring women with high risk pregnancies.