The stability of the mRNA involved in the inductive penicillinase synthesis of Bacillus cereus 569 was examined by different methods. Experiments on systems with high levels of actinomycin D were inconclusive owing to extensive lysis of the cells. In the presence of lower concentrations of actinomycin D, the mRNA of penicillinase appeared stable. In the absence of penicillin and phosphate inducers, the fate of the mRNA was characterized by a two-phase curve. On withdrawal of the inducer, the messenger underwent no decomposition for 0-10 min, but after 10 min it started to break down with a half-life of 10-15 min, depending on the conditions of the experiment.