Pregnancy-specific beta 1-glycoprotein (SP1) concentrations were measured by nephelometry in 133 serum samples from 74 women in their last trimester of pregnancy. All women carried one child but their pregnancies were complicated by pre-eclampsia, essential hypertension, diabetes mellitus or rhesus isoimmunization. Most of the women with pre-eclampsia or essential hypertension who gave birth to infants of normal weight had SP1 values evenly distributed within the reference range. Women with insulin-dependent diabetes mellitus who gave birth to infants to normal birth weight or large-for-date infants had SP1 levels well within the reference range and even showed a tendency to overrepresentation above the geometric mean. Women with rhesus isoimmunization all delivered infants of normal birth weight. Their SP1 values were near or above the geometric mean of the reference range. No special pattern of SP1 levels was observed in relation to the severity of the immunization.