Résumé : An increase before lysis in the optical density of cultures of Escherichia coli has been observed with ampicillin or azlocillin when concentrations were higher than the minimal inhibitory concentrations. With ampicillin that increase in optical density was less marked as the concentration increased. With azlocillin, however, the increase in optical density showed little relationship with the antibiotic concentration. By destroying the antibiotic activity with a beta-lactamase, after 30 min., one, two, three and four hours of contact, it was shown that the delay for culture regrowth depended on the ampicillin concentration, not on the azlocillin concentration. In conclusion, a relationship was established between the early response of the growth curve of Escherichia coli to various concentrations of ampicillin and azlocillin and the delay of regrowth after enzymatic inactivation of both antibiotics.