Concentrations of polyamines, DNA and oestrogen receptor (ER) binding sites have been estimated in 23 malignant and 2 benign breast tumours. Polyamine and DNA concentrations, expressed per unit wet weight of tissue, were significantly higher in ER-poor (less than 10 fmol/mg cytosol protein) than in ER-rich tumours. Spermidine/spermine ratios were similar in ER-poor and ER-rich tumours. There was a relationship between polyamine and DNA levels and when polyamine concentrations were expressed per unit weight of DNA those in ER-poor tumours did not differ from those in ER-rich tumours. Possible reasons for these findings are discussed.