A method is described for the determination of tamoxifen in serum. The drug was extracted from the serum and separated from its metabolites by thin-layer chromatography. Irradiation of the thin-layer plate with ultra-violet light after development converted tamoxifen to a fluorescent product which could be estimated in situ by densitometry. The fluorescent product was identified as a substituted phenanthrene by thin-layer chromatography, and gas chromatography–mass spectrometry. The method allowed concentrations as low as 6·8 nmol/l serum to be measured. After an oral dose of tamoxifen (10 mg) to a female patient the maximum serum concentration (66·2 nmol/l) was achieved in 3 h. When 10 mg tamoxifen were taken twice daily for 21 days by the same patient serum concentrations of about 500 nmol/l were achieved.