[en] The induction of the Staphylococcus aureus BlaZ and Bacillus licheniformis 749/l BIaP beta-lactamases by beta-lactam antibiotics occurs according to similar processes. In both bacteria, the products of the blal and blaR1 genes share a high degree of sequence homology and act as repressors and penicillin-sensory transducers respectively. It has been shown in S. aureus that the Blal repressor, which controls the expression of BlaZ negatively, is degraded after the addition of the inducer. In the present study, we followed the fate of Blal during beta-lactamase induction in B. licheniformis 749/l and in a recombinant Bacillus subtilis 168 strain harbouring the pDML995 plasmid, which carries the B. licheniformis blaP, blal and blaR1 genes. In contrast to the situation in B. licheniformis 749/l, beta-lactamase induction in B. subtilis 168/pDML995 was not correlated with the proteolysis of Blal. To exclude molecular variations undetectable by SDS-PAGE, two-dimensional gel electrophoresis was performed with cellular extracts from uninduced or induced B. subtilis 168/pDML995 cells. No variation in the Blal isoelectric point was observed in induced cells, whereas the DNA-binding property was lost. Cross-linking experiments with dithiobis(succimidylpropionate) confirmed that, in uninduced recombinant B. subtilis cells, Blat was present as a homodimer and that this situation was not altered in induced conditions. This latter result is incompatible with a mechanism of inactivation of Blal by proteolysis and suggests that the inactivation of Blal results from a non-covalent modification by a co-activator and that the subsequent proteolysis of Blal might be a secondary phenomenon. In addition to the presence of this co-activator, our results show that the presence of penicillin stress is also required for full induction of beta-lactamase biosynthesis.