We have studied strains of enterobacteria which are resistant to first generation cephalosporins, in relation to their cephalosporinase production. Therefore, this is not an epidemiological study. For these selected strains, if a difference in MIC greater than or equal to 2 base 2 logarithms is considered to be significant, Moxalactam appears as more potent than cefotaxim. An appropriate study shows that these findings are related to the production of cephalosporinase. If very little cephalosporinase is produced, cefotaxim is more potent, implying better accession to the lethal target of the bacteria. Conversely, when cephalosporinase production is higher, Moxalactam is more potent. Moxalactam can be shown to be more resistant to hydrolysis by cephalosporinases.