The assay of estriol is of interest in biological samples such as serum. During pregnancy estriol becomes the dominant estrogen and serum levels increase from 0.3 ng/mL to above 8.0 ng/mL. Its main use is in the diagnosis of foetal diseases. High affinity antibodies are useful in facilitating immunoassays capable of measuring such low levels. Assay designs are likely to benefit from the use of estradiol-6-X (ie E2) reagents or other nonestriol conjugates.