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Summary A specific and sensitive, double antibody, solid phase radioimmunoassay was developed for measurement of placental protein 5 (PP5) in serum. A sensitivity limit of 0.8 μg/1 was achieved by pre‐incubation with antiserum and use of a small assay volume. Single serum samples from 244 retrospectively proven normal pregnancies were used to construct a normal range of PP5 concentrations. PP5 was detectable at week 7 and rose to reach a maximum mean value of 54 μg/1 at weeks 36–37. Diurnal variation of PP5 concentration was similar to that encountered from day to day, and slightly greater than variation due to the radioimmunoassay. The decline of serum PP5 concentration post partum was initially rapid (half‐life 5–14 minutes), but slowed after 0.5–1 hours; immunoreactive PP5 was still detectable seven days after delivery.

Summary Maternal serum PP5 concentrations were assayed in pregnancies complicated by intrauterine growth retardation, pre‐eclampsia, diabetes mellitus, placental abruption and twins. Elevated PP5 levels were noted in twin pregnancies and to a lesser extent in diabetes and pre‐eclampsia. Measurement of PP5 was of no value in the detection of fetal growth retardation. The great majority of PP5 levels above the normal range were associated with a placental abnormality.

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