Low concentrations of antibiotics have been shown to alter the morphology and ultrastructure of bacteria. Exposure to beta-lactam antibiotics produces large Gram-positive cocci or long filaments of Gram-negative bacilli. The ultrastructure of staphylococci in infected animals and patients treated with beta-lactam antibiotics is comparable to the structure of staphylococci grown on a solid support medium such as hard agar or a filter membrane but is different from the structure of staphylococci grown in liquid media. Antibiotic-modified bacteria are phagocytosed very efficiently; considering their bacterial mass, Escherichia coli filaments are killed much more efficiently by PMNs than is an equal mass of normal sized bacilli. Antibiotics at sub-MIC alter the synthesis and excretion of bacterial metabolites which results in a change in their virulence. Antibiotics at a very low dosage, 10 mg of ampicillin per day which resulted in sub-MICs in the urine, were shown to cure urinary infections in patients. These therapeutic results were attributed to the inhibition of bacterial adherence by sub-MICs of ampicillin.