The stability of a preparation of lyophilised serum and its suitability for use as a reference material for routine assays of cortisol, oestradiol, and progesterone have been studied in four laboratories with a variety of assay systems. Cortisol and oestradiol were also measured by gas chromatography-mass spectrometry. The lyophilised serum was suitable for use in all routine and reference assay methods examined, with between-method variability no greater than that for frozen serum pools. The concentrations of cortisol, oestradiol, and progesterone were predicted by accelerated degradation studies to decline by 0.01% per annum if the preparations were to be stored at --20 degrees C. The testosterone content of the preparation, determined in one laboratory, provided no evidence for degradation. The preparation can be shipped for use at ambient temperature without deleterious effect.