Novel 1-substituted 3-(p-isopropylphenyl)-5-phenylformazans (3a-g) were synthesized and characterized by elemental analysis, H-1 NMR, and FT-IR techniques and UV-visible spectroscopy. Antiproliferative activities of 3a-g against HeLa and C6 cells were determined using the BrdU cell proliferation ELISA assay. 5-Florouracil was used as the positive control. The effects of substituents (-H, -CH3, and A) and their positions (ortho, meta, and para) on the antiproliferative activities were evaluated. The results of the assay indicate that I substituent exhibited higher activity against the cells at the meta and para positions than -CH3 and -H substituents. 3a-g exhibited both high antiproliferative activities against C6 cells and noncytotoxicity. 3a-g may be anticancer drug candidates.