The small explants (0.5mm³ cuboid) of sarcoma induced by MC in C3H/HeN mouse were transplanted syngeneically under the serosa of the spleen. The recipient mice were devided into two groups. One was young group (two-month-old) and the other was old one (eleven-month-old). Transplanted tumor tissue survived in all recipient mice. Seven days after transplantation tumor tissue grew protruding from splenic surface and also infiltrated deeply into splenic parenchyma. Fifty three days after transplantation, the whole spleen was replaced by conglomerated tumor mass. Tumor growth was more conspicuous in young group of mice than in old one. Histologically, the boundary between transplanted tumor tissue and splenic parenchyma was progressively cleared, but any membraneous structure was not ultimately formed. Many plasma cells and lymphocytes were directly contact with tumor cell and no specialized cellular struture was formed between them. In part which showed tumor cells and lymphocytes were in direct contact, simultaneously plasma membrane of tumor cell became unclear. There were no prominent histological difference between young group of mice and old one.