A study was made of the in vivo effect of the anti-oestrogen tamoxifen on the growth and cell cycle kinetics of the oestrogen and progesterone receptor-positive T61 human breast carcinoma and of the oestrogen and progesterone receptor-negative T60 human breast carcinoma grown in nude mice. The T61 tumour was exposed to single doses of 0.1, 1.0, 5 or 10 mg tamoxifen, resulting in serum concentrations 1 day after treatment ranging from less than 27 to 134 nM. The T60 tumour was exposed to a single dose of 1.0 mg tamoxifen. The effect on the tumour growth curves was determined according to a transformed Gompertz function, and the effect on the cell cycle distributions was estimated by flow cytometric DNA analysis on tumour tissue obtained by fine-needle aspirations at intervals after the treatment. The results showed that in the T61 tumour, tamoxifen induced a dose-related growth inhibition, whereas it had no concomitant effect on the cell cycle distribution of the tumour. The treatment had no effect on the T60 tumour. The results indicate that tamoxifen is unsuitable as a synchronizing agent in human breast tumours with responses to the drug similar to those of the T61 and T60 breast tumours.