The clonal growth of gastric carcinomas was investigated immunohistochemically in C3H<==>BALB/c chimeras using a strain specific antibody. C3H, BALB/c and chimeric mice were given N-methyl-N-nitrosourea 0.5 mg/mice once a week for a total of 10 times by intragastric intubation and observed until week 50. In normal gastric mucosa of the chimeras, each gland was composed entirely of C3H strain specific antigen (CSA)-positive or -negative cells and no mixed glands were found. Cells of all adenomatous hyperplasias and adenocarcinomas in chimeric mice were, in each case, homogeneous for one or other of the parental types, while comprising both surface mucous cell and pyloric gland cell forms. The results clearly suggest that individual cancers are derived from single cells with multi-potential activities and that cellular differentiation of gastric cancer cells occurs secondarily.