Four Klebsiella pneumoniae strains, isolated in two geographically distant French hospitals, were found to produce constitutive beta-lactamases with an unusual isoelectric point for this species (8.1). The four enzymes were chromosomally encoded and related to the Ambler's class A plasmid-mediated SHV-type enzymes. The four enzymes were predominantly penicillinases, with good activity against amino- and ureidopenicillins. They poorly hydrolysed cephalothin, cephaloridine and cefoperazone and did not hydrolyse third-generation cephalosporins and aztreonam. The four enzymes were susceptible to the inhibitory effect of clavulanic acid. Finally, the strains were all found to produce an acetyl-esterase. The acetyl-esterases catalysized hydrolysis of the acetoxy group of cephalothin and cefotaxime although this did not affect their antibacterial activities. These esterases were not susceptible to the inhibitory effect of clavulanic acid and EDTA.