Abstract. Changes in morphology and cell kinetics are described in a rat thyroid transplantable tumour (TTT) during the first few transplant generations. The growth of TTT in animals was possible only with an increased circulation level of the thyroid stimulating hormone (TSH). With serial transplantation subcutaneously in isologous animals, the morphology of TTT changed dramatically from that of a follicular tumour in the 3rd passage to become, by the 9th generation, a poorly differentiated tumour with a trabecular arrangement of cells. This change in tumour morphology was accompanied by an increase in the number of proliferating cells–mitotic index (MI), [3H]thymidine labelling index (LI), growth fraction (GF)–and cell loss factor (O) as well as a decrease in the cell cycle time (Tc) and potential population doubling time (TPD). TTT belongs to the class of tumours with a low proliferative activity and might be used in a variety of cell kinetic, radiobiological and chemotherapy studies.