The "Autobac" apparatus permitted the rapid determination of the light-scattering index 50% (LSI50) and inhibitory concentration 50% (CI50) of eight antibiotics -- kanamycin, amikacin, lividomycin, gentamicin, tobramycin, sisomicin, netilmicin and colistin -- for the strain of Escherichia coli ATCC-25922. The results were compared with those of the reference method using an agar dilution method. The percentage of inhibition (% I) was obtained from LSI by applying the formula % I = 10(-(GIXLSI)). The growth index (GI) has an influence on the values of the rapid method. The best results were observed in the GI ranges of 1.2 to 1.7. For all the antibiotics except tobramycin and colistin, the CI50 values of the two methods were very similar. The values of LSI50 were not assimiliated to those of CI50. These three parameters had a small variability. The averages of standard deviation were 0.23 log2 for the CI50 of the reference method, 0.24 log2 for the CI50 of the rapid method and 0.32 log2 for the LSI50. The "Autobac" enables the determination of CI50 by a simpler and faster technique.