The results of in vitro tests against solid tumors have been generally influenced by the mixing in of fibroblasts, and so have SDI tests. We demonstrated that the influence of fibroblasts can be excluded by performing the SDI test on an agar layer, thus significantly inhibiting the growth of fibroblasts in the liquid top layer. In the agar SDI test, SDS is used to dissolve formazan; however, this lowers the sensitivity to some extent. We added additional DMSO to SDS and to raise the sensitivity.