Amoxicillin (α-amino-p-hydroxybenzyl penicillin, BRL 2333) is a new semisynthetic penicillin which is structurally similar to ampicillin, but which is better absorbed and yields higher concentrations in serum and urine. The in vitro susceptibility of 145 strains of Enterobacteriaceae and 30 isolates of Pseudomonas aeruginosa was determined against various concentrations of amoxicillin and ampicillin in agar. In addition, inhibitory zones around discs containing 10 μg of amoxicillin were measured and compared with results of agar dilution studies. The drug also was evaluated in the treatment of 38 patients with bacteriuria, who received doses of either 750 mg or 1 g/day for 10 days. In vitro, amoxicillin was comparable in activity to ampicillin; most isolates of Escherichia coli and Proteus mirabilis were inhibited by 10 μg or less/ml, whereas the majority of strains of Enterobacter, Klebsiella, indole-positive Proteus species, and Pseudomonas grew in concentrations greater than 50 μg/ml. Clinically, amoxicillin was effective in eradicating bacteriuria due to susceptible organisms and was very well tolerated. For practical purposes, however, amoxicillin performed no better than a host of other drugs presently available for the treatment of acute, uncomplicated bacteriuria.