Subinhibitory concentrations of antibiotics can produce in vitro aberrant forms of bacteria that are similar to those observed in specimens and cultures from patients being treated with antibacterial agents. Eight species of bacteria were grown on membranes placed on agar containing subinhibitory concentrations of nine antibiotics. The resulting organisms were examined by Gram stain and electron microscopy. Gram stains showed filamentous and granular forms of enterobacteria with bipolar staining, giant staphylococci, and rodlike pneumococci. Electron micrographs showed changes in the number and distribution of ribosomes in enterobacteria and septum abnormalities in cocci. Such abnormal forms can occasionally simulate the appearance of quite different species, and they may indicate the presence of a subinhibitory antibiotic concentration at the site of infection as a result of prior antibacterial therapy.