3-Diazopyrroles, a class of compounds particularly interesting from a chemical and biological point of view, were assayed for their ability to induce gene mutations employing back mutation (his+ reversion) test in the philamentous bacterium Streptomyces coelicolor at various time during life cycle. Our results suggest that in evaluating the mutagenicity and toxicity of chemicals in Streptomyces system it is important to consider factors such as growth phase. Furthermore in this series of diazopyrroles a relationship between toxicity, mutagenicity and chemical structure was found. The observed mutagenic activity can be the molecular basis for the appearance of antitumor activity.