The effect of antibacterial treatment on the prevalence of gram-negative rods (enterobacteria and Pseudomonas) in the mouths of patients with leukemia was studied. Of the patients' salivary samples, 45% contained gram-negative rods compared with 12% in a control group of healthy persons. The antibacterial resistance patterns of gram-negative rods from healthy persons were different from those of the hospitalized patients. No relationship was found between the amount of antibacterial treatment and presence of gram-negative rods. The gram-negative rods recovered from individual patients were usually not resistant to the antibacterial agents given to the patients in question. Only a few patients harbored the same clone of a gram-negative rod for a lengthy period. The resistance patterns of these clones of gram-negative rods were similar to those of gram-negative rods that were transient in the mouth. It was concluded that resistance to antibacterial agents is not a factor of major importance for the capacity of the gram-negative rods to become established in the mouths of hospitalized patients with leukemia.