Determination of minimal inhibitory concentrations (MICs) provides data on the susceptibility or resistance of a bacteria; however, in susceptible bacteria this parameter is not predictive of effectiveness of the antimicrobial agent. Bactericidal activities of cefadroxil, of amoxicillin, and of the amoxicillin-clavulanic acid combination on bacteria commonly found in ENT and lower respiratory tract disease were studied comparatively. The antibiotics were given by the oral route to six healthy volunteers. With beta-lactamase-producing and non-beta-lactamase-producing strains of Escherichia coli, amoxicillin produced MICs consistent with susceptibility but failed to exhibit a bactericidal effect, whereas cefadroxil was bactericidal. Combination of amoxicillin with an inhibitor did not modify this activity on E. coli and failed to produce a bactericidal effect on Klebsiella pneumoniae similar to that seen with cefadroxil. Amoxicillin with and without clavulanic acid exhibited comparable effectiveness on Streptococcus pyogenes and S. Pneumoniae. The bactericidal effect of cefadroxil on S. pneumoniae was of similar magnitude but shorter duration than that of amoxicillin. Cefadroxil and the amoxicillin-clavulanic acid combination had similar bactericidal effects against Staphylococcus aureus. These antibiotics exhibited a time-dependent effect on Gram positive microorganisms. These pharmacodynamic data, together with measures of bactericidal activity, may be very helpful for selecting the appropriate antibiotic and dosage.