In this improved assay only a 500-microL sample is needed and a single assay can be done within 30 min, 10 samples within 90 minutes. The sample of serum or plasma is diluted 20-fold with water, and the estriol is adsorbed from it onto graphitized carbon black ( Carbopack B, Supelco ). After two washings the estriol is desorbed with chloroform/methanol (60/40 by vol), which then is evaporated. The residue is redissolved in 50 microL of water/acetonitrile, and 20 microL is injected into the chromatograph. Analytical recovery for estriol-supplemented serum or plasma averaged 98.6%. Day-to-day CVs ranged from 3.9% at 2 micrograms/L to 2.1% at 20 micrograms/L. The limit of sensitivity is 0.3 micrograms/L, which makes this procedure suitable for determination of estriol even in the first half of pregnancy. Our method is inexpensive, and shows that liquid chromatography can be used to determine estriol in pregnancy serum or plasma. It also is more sensitive and precise and requires less sample than other such methods.