Serial measurements of serum oestriol (E3) were made in 56 normal pregnancies from 10 to 40 weeks gestation using a radioreceptor assay. Several parameters of the assay and its intrapatient variability were examined. The mean curve for pregnancy E3 levels and limits of normal variation have been identified and compared with those of other series. A positive correlation was found to exist between infant birth weight and the last E3 value in normal pregnancies prior to the onset of labour. Advantages of serum E3 assays over urinary E3 determinations are discussed.