Although many kinds of chemosensitivity test are available for the selection of suitable anticancer agents, the results of in vitro tests against solid tumors have been generally influenced by mixed in fibroblasts, as 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. The sensitivity of the agar SDI test was determined on the 3-4th day after incubation because cell proliferation tended to be delayed, and there were also somewhat higher cell counts of about 40,000 per well. The inhibition indices with and without agar in vitro were the same, showing no significant differences. With nude mice, the transplanted tumor index value of the agar SDI test is higher than that of SDI, and approaches in value the SDI of a pure tumor cell, which means that the sensitivity of a solid tumor might be more accurately shown by an agar SDI test than by an SDI test.