It is well known that urodeles have the most powerful regenerative capacities among vertebrates, but there is little realisation that they are extremely resistant to spontaneous or chemically induced tumours. Regeneration and carcinogenesis have been considered to be two sides of the same mechanism. Since antagonism between regeneration and carcinogenesis was expected in previous studies, the present study was intended to clarify this relationship in greater detail by changing the amounts of carcinogen stepwise. When 1 microliter nickel subsulfide solution was administered in various amounts (1 microgram/microliter approximately 40 micrograms/microliter) into lentectomized newt eyes, the delay of initiation in lens regeneration for 6 months and an increased inhibition rate of lens regeneration at 6 months were observed in proportion to the increase in carcinogen dosage. The tumour production rate increased in accordance with the increase in the amounts of carcinogen. The conspicuous result obtained in the present study was that lens regeneration from dorsal iris and tumour induction from ventral iris occurred simultaneously in the same eye after administration of moderate amounts (10 micrograms/microliter) of carcinogen. These data clearly indicated that the regenerating dorsal iris is persistently resistant to carcinogen, whereas the ventral iris, which cannot regenerate lens, is susceptible to tumour induction. This strongly suggests that the lens regeneration system in the newt has special advantages for research on the relationship between regeneration and carcinogenesis.