In this review we have compared and contrasted the antibacterial and pharmacological properties of the aminopenicillins currently available. It is apparent that there is little hard evidence (except in the case of thphoid fever) that either amoxycillin or the pro-drug esters of ampicillin are significantly more effective than ampicillin itself, despite their better absorption and higher serum levels. However, amoxycillin and the ampicillin pro-drug esters do cause fewer bowel side-effects than ampicillin, which may justify their use despite the extra cost involved. The problem of susceptibility to penicillinases is unlikely to be solved by modification of the aminopenicillin molecule. A more likely solution is administration of these substances with beta-lactamase inhibitors such as clavulanic acid. Trials with such combinations are already under way.