Cell killing activities of ferrous and ferric irons (Fe(2+), Fe(3+)) in cultured 3T3 cells were investigated by the colony forming method in the culture medium containing 10% calf serum. In the colony forming method, small isolated colonies are seen in the dish which are the resulted of individual cells having undergone a series of cell divisions. Survival curve for Fe(2+)-induced loss of reproductive capacity (cell death) in 3T3 cells, showed a shoulder at low concentrations and only became exponential at high concentrations. Cells affected at 0.05mM Fe(2+) underwent apparently normal cell divisions, but at a much slower rate than the control. Survival rate at 0.5mM Fe(2+), which iron was added after seeding cells (added group), showed 0.75% but that at 0.5mM Fe(3+) did 19.3%. From the determination of Fe(2+) content carried out by the colorimetric method used N-PSAP, the oxidation of Fe(2+) was seen rapidly after adding Fe(2+) into the culture medium. In the cases of cells seeded 3 hours after adding Fe(2+) or Fe(3+) (preadded group), the survival rates showed much lower than those of the added groups, that is 55% at 0.5mM Fe(2+) and 89% at 0.5mM Fe(3+). The facts suggest that the radical reactions iniciated by the oxidation of Fe(2+) under the presence of cells, occured just after adding Fe(2+), play a crucial role in the damages on the adhesion of cells onto the dish and the following proliferation of cells. Serum lipid peroxides, as thiobarbituric acid reactive substances (TBA value), of the final cultured medium at colony counting time were measured. In the Fe(2+) added group, the TBA values more increased in proportion to the added concentrations of Fe(2+), whereas in the Fe(3+) added group, the increments of TBA values were very slightly and not to be depended on the concentrations. Furthermore, the TBA values of the Fe(2+) or Fe(3+) preadded groups were estimated as well as those of the added groups, respectively. The facts suggest that serum lipid peroxides induced by Fe(2+) have little effect on the proliferation capacity of cells in the level of cell culture.