Epithelial-like cells originating from 10-day and 8-week old BD rats were treated in vitro with dimethylnitrosamine (DMN) and N-methyl-N'-nitro-nitrosoguanidine (MNNG). Morphologically, no differences were seen between treated and untreated cells up to the time when the cells were transplanted into syngeneic hosts. However, the treated cells grew in soft agar and once injected subcutaneously or intraperitoneally into newborn rats, developed tumours after a latent period of 9-12 weeks. The tumours obtained with DMN-treated cells were well differentiated adenocarcinomata, whereas carcinosarcomata were observed with the MNNG-treated cells.