Daily intravenous administration of 50 mg/kg cyclosporin A (CsA), after excision of cheek pouch tumours induced with 9,10-dimethyl-1,2-benzanthracene (DMBA), enhanced the incidence of cervical lymph node metastasis to 93%, a significant increase over the rates observed for animals receiving a dosage of 25 mg/kg (43%), and for control animals receiving no CsA (40%). No significant effect was evident on the growth rates of the metastasized tumour cells in the lymph node.