Thank you very much for your letter regarding my letter to the editor and for the responses from Drs. Eichenwald and Mortimer. One interesting development that could be added as an addendum to show the artificial atmosphere of in vitro sensitivity is the finding that since we stopped incubating our sensitivity plates in a CO2 atmosphere, we have noted that only 11.5% of E. coli are resistant to kanamycin and only 8% resistant to ampicillin. Our sensitivity media is Mueller-Hinton with a pH of approximately 7.4. Apparently incubatioii in a CO2 atmosphere, which decreases the pH of the media, interferes with the sensitivity of E. coli in particular to kanamycin and ampicillin.