Amdinocillin was studied alone and in combination with three other beta-lactam antibiotics (aztreonam, cefoperazone, and ceftriaxone) for activity against gram-negative bacilli. These antibiotic combinations failed to show synergy by the checkerboard double-dilution test or by killing kinetic studies. However, amdinocillin did show additive killing action when combined with the other beta-lactam antibiotics studied. Amdinocillin failed to induce or inhibit beta-lactamase production in species of Enterobacteriaceae, but with Pseudomonas aeruginosa, beta-lactamase production was induced. It is concluded that the activity of amdinocillin alone or in combination with another beta-lactam antibiotic should not be more effective in the treatment of infections by gram-negative bacilli than just using the beta-lactam antibiotic alone at a higher dose.