The present invention relates to novel assays for assessing the degree to which a cell is metabolically active. Also provided, are mutagenicity assays and, more specifically, methods for determining whether a given agent is genotoxic. Embodiments of the assays employ a microorganism, or a mammalian cell, that has been genetically modified to produce light when the presence of test agent results in a mutation, e.g., reversion or forward, in the DNA of such microorganism or cell.