Automatic graphic display of the data output increases the effectiveness of computer analysis of results of hospital antimicrobial-susceptibility tests. The system we used plots the frequency distribution of inhibition-zone diameters around any antibiotic disk for selected groupings of bacterial isolates. It tallies the group's relative frequency of isolation and its percentage susceptible to the antibiotic. These plots allow comparison of differences due to variables, such as time and source of isolation. They permit detection of distinctive subgroups among the hospital's flora and of associated resistance to multiple antibiotics. They can contribute to quality control of the susceptibilitytesting procedure and of the process of bacterial classification itself. They provide for the practicing physician a concise and graphic statistical basis for antibiotic selection.