This invention is an assay to evaluate growth inhibition for a wide range of agents on mammalian and potentially other cell types. Live cells are seeded on micron scale wells, grown in optimal culture conditions, and expose to different test conditions. In normal, nontoxic conditions, cells are expected to double every 24 hours, producing colonies. However, cells exposed to agents that inhibit cell division or induce cell death give rise to smaller or non-existent colonies, depending on the level of toxicity. The number of cells in a colony is estimated by measuring total DNA per well, which is stained with a dye. Cell growth is calculated by comparative colony size distribution. An automated system that takes images is able to capture up to 100 colonies at the time. Image analysis is also automated with a custom software, which processes thousands of colonies in seconds.