Papillary superficial and nonpapillary invasive bladder carcinomas of humans are two disease entities exhibiting completely different biological behavior. Studies were performed on the susceptibilities of various strains of mice to induction of bladder carcinomas by N-butyl-N-(4-hydroxybutyl)nitrosamine (BHBN), and the type of carcinomas that developed. BHBN at concentrations of 0.05% and 0.01% in drinking water was given to female mice of strains A/Jax, AKR/Jax, C3H/He, DBA/2, and C57BL/6 for 22 weeks. The incidence of nonpapillary invasive bladder carcinomas in these strains was 40%, 100%, 100%, 89%, and 89%, respectively, in groups treated with 0.05% BHBN and 30%, 60%, 60%, 90%, and 40%, respectively, in groups treated with 0.01% BHBN. At both levels BHBN and in all strains the bladder carcinomas induced were of the nonpapillary invasive type.