This paper presents some observations on the C(3)H-Hei strain cells, established from spontaneous mammary tumor of the C(3)H mouse. 1) The C(3)H-Hei strain has been propagated over 800 days ever since December 1961. 2) The most optimal medium consist of 50% bovine serum, 0,25% lactalbumine hydrolysate, 0,05% yeast extract and Earle's saline. 3) The primary culture and early subculture of this cell strain morphologically exhibited a number of cell sheets of epithelial-like cells, but suddenly at the 4th generation. they have transformed the morphological pattern from the epithelial-like cells to the fibroblast-like cells of a uniform size. 4) Tbe growth rate in cell population was about 8 folds in 6 days. 5) Chromosome number of the Hei-strain showed a higher pattern ranging from 64 to 72. 6) The tumor-inducing capacity of this strain was examined by subcutaneous back-inoculation into C3H, Zb and Cb mice. A certain portion of the mica had a total body irradiation (400r) before inoculation and it was found that 100% C(3)H mice challenged before injection had developed tumors, but Zb and Cb mice did not.