4,4'-Diaminodiphenylmethane (4,4'-methylenedianiline) (DDPM) promoted the development of thyroid tumors in rats treated with a subeffective dose of N-bis(2-hydroxypropyl)nitrosamine (2,2'-dihydroxy-N-nitrosodipropylamine) (DHPN) for thyroid tumorigenesis. Male inbred W rats were given a single ip injection of 280 mg DHPN/100 g body weight and fed diets with or without 1,000 ppm DDPM. Thyroid tumor incidences at the end of week 20 of the experiment were 90% (19/21) in rats given DHPN and then DDPM and 28% (6/21) in rats given DHPN alone. The incidence of thyroid cancers was 9.5% (2/21) in rats first given DHPN and then DDPM. Untreated rats and rats given DDPM alone had no thyroid tumors after 20 weeks. Incidences of kidney tumors were 38% (8/21) in rats given DHPN and then DDPM and 28% (6/21) in rats given DHPN alone. No tumors were found in the kidneys and lungs of rats given DDPM alone and in those of control rats. Treatment with DDPM alone slightly but not significantly decreased the serum concentrations of thyroxine and triiodothyronine; treatment with DHPN plus DDPM had no such effect.