Proliferation rates of leukemia and normal cells exposed to OSCs. We measured the biological activity of OSCs in normal fibroblast diploid (WI38) and leukemic (AML-3) cells using the MTT assay. (A) Differential activity was observed for a subset of compounds. Group I compounds (F2, F3, and H1) are biologically inactive, Group II compounds (F1, H2, H5, H6 and N2) reduce proliferation in leukemic cells only and Group III compounds (F4, F5, F6, F7, F8, H3, H4, H7, H8 and N1) affect proliferation of both leukemic (AML-3) and normal, non-transformed cells (WI38) to varying degrees. (B-D) Representative analyses of compounds from each group: each compound was assayed in three independent experiments. AML-3 cells are shown with squares; WI38 cells are shown with triangles.