The bactericidal effect of various paired combinations of 11 antimicrobial agents on 30 strains of Klebsiella-Enterobacter-Serratia was examined. The commonest effect found was indifference, but synergism and antagonism were also seen. Unexpected results were antagonism between two bactericidal drugs, and synergism between bactericidal and bacteriostatic agents. Thus, we found antagonism between two aminoglycosides, between two β-lactam antibiotics, and between aminoglycosides and β-lactams. Both antagonism and synergism were found between chloramphenicol and β-lactams and between chloramphenicol and aminoglycosides. The outcome of specific combinations was different on different strains of the three genera, and the results were completely unpredictable.