Sanfetrinem is a trinem -lactam which can be administered orally as a hexatil ester. We examined whether its -lactamase interactions resembled those of the available carbapenems, i.e., stable to AmpC and extended-spectrum -lactamases but labile to class B and functional group 2f enzymes. The comparator drugs were imipenem, oral cephalosporins, and amoxicillin. MICs were determined for -lactamase expression variants, and hydrolysis was examined directly with representative enzymes. Sanfetrinem was a weak inducer of AmpC -lactamases below the MIC and had slight lability, with a k_cat of 0.00033 s¹ for the Enterobacter cloacae enzyme. Its MICs for AmpC-derepressed E. cloacae and Citrobacter freundii were 4 to 8 µg/ml, compared with MICs of 0.12 to 2 µg/ml for AmpC-inducible and -basal strains; MICs for AmpC-derepressed Serratia marcescens and Morganella morganii were not raised. Cefixime and cefpodoxime were more labile than sanfetrinem to the E. cloacae AmpC enzyme, and AmpC-derepressed mutants showed much greater resistance; imipenem was more stable and retained full activity against derepressed mutants. Like imipenem, sanfetrinem was stable to TEM-1 and TEM-10 enzymes and retained full activity against isolates and transconjugants with various extended-spectrum TEM and SHV enzymes, whereas these organisms were resistant to cefixime and cefpodoxime. Sanfetrinem, like imipenem and cefixime but unlike cefpodoxime, also retained activity against Proteus vulgaris and Klebsiella oxytoca strains that hyperproduced potent chromosomal class A -lactamases. Functional group 2f enzymes, including Sme-1, NMC-A, and an unnamed enzyme from Acinetobacter spp., increased the sanfetrinem MICs by up to 64-fold. These enzymes also compromised the activities of imipenem and amoxicillin but not those of the cephalosporins. The hydrolysis of sanfetrinem was examined with a purified Sme-1 enzyme, and biphasic kinetics were found. Finally, zinc -lactamases, including IMP-1 and the L1 enzyme of Stenotrophomonas maltophilia, conferred resistance to sanfetrinem and all other -lactams tested, and hydrolysis was confirmed with the IMP-1 enzyme. We conclude that sanfetrinem has -lactamase interactions similar to those of the available carbapenems except that it is a weaker inducer of AmpC types, with some tendency to select derepressed mutants, unlike imipenem and meropenem.