Simple, rapid colorimetric tests for lysogenic induction (the derepression of a latent bacterial virus) are described. A quantitative test and a more rapid semiquantitative test are based on the assay of the beta-galactosidase synthesized from lacZ gene fused to an operon under lambda repressor control. These biochemical "inductests" are suitable for screening programs designed to detect agents that damage DNA and that are of potential interest in carcinogenesis and cancer chemotherapy.