The present study has been undertaken to pursue the cytotoxic effects of cis-Platin on the osteoid formation in metaphysis of rat tibia. By using the immunohistological staining method for type I collagen in rat tibial osteoid, the author detected the deposition of type I collagen, which is the collagenous constituent of endochondral osteoid, after administration of cis-Platin in experimental animals. For the immunological reactions of type I collagen, we used the rabbit anti-rat collagen type I polyclonal antibody as primary antibody and biotinylated goat anti-rabbit IgG as secondary antibody. The distributions of immunohistological reactions in the each of metaphyseal osteoids were analyzed with an image analyzer, and we studied the variances of type I collagens by statistical probabilities. In 12 hours after cis-Platin injection, immunoreactive area in the osteoid of metaphysis was distinctly decreased. Immunoreactive area of type I collagen in osteoids of 1 day and 3 days group metaphysis was increased more than that of 12 hours group and the type I collagen in the metaphysis showed weak immunoreactions of type I collagens with an image analyzer. In the osteoids of 7 days group after cis-Platin injection, the immunoreactive area was similar to that of control group. It is consequently suggested that cis-Platin would induce the decrease of type I collagen in the osteoid. But the type I collagen in tibial osteoid shows the increase from a few days after cis-Platin injection.