To develop a simpler method of performing the collagen gel droplet-embedded culture drug sensitivity test (CD-DST), we examined the introduction of colorimetric quantitative determination of images for evaluation of anticancer effect against cancer cells alone in the presence of fibroblasts, based on differences in proliferative morphology and stainability with neutral red of cells within collagen gel drops determined using a video-microscope and NIH Image software. In examinations using a human cancer cell line and a fibroblast cell line, a high degree of linearity between number of cancer cells and image-optical density was found within the range of 10(2) - 10( 6) cells / droplet (r (2) = 0.933). Using NIH Image, fibroblast cells could be eliminated at a cut-off value of 128, and an immunocytochemical method demonstrated that the cells eliminated from the image were indeed fibroblasts, and those remaining were cancer cells. CD-DST was carried out with mixtures of cancer cells with fibroblasts at various ratios, and the feasibility of evaluating anticancer activity in cancer cells alone with no effect of fibroblasts at any mixing ratio was confirmed. In addition, for CD-DST of primary cell cultures of human lung cancers collected at the time of surgery, a high correlation between results obtained with the volume supplementation method, a current cell quantification method, and those with the imaging colorimetric quantification method was obtained (r = 0.933). These results indicate that introduction of imaging colorimetric quantification utilizing NIH Image makes CD-DST a quick and simple method that should be highly useful for clinical chemosensitivity testing using primary cell cultures of human cancers.