The in vitro interactions of four beta-lactam antibiotics and five antineoplastic agents were examined with 100 clinically isolated strains of four species of gram-negative bacilli. Generally, by the checkerboard dilution method, beta-lactam antibiotics, when tested in combination with mitomycin C, bleomycin, or 5-fluorouracil, showed synergistic action, whereas when tested in combination with carboquone, they showed antagonistic action. Almost no combinations of adriamycin showed the interactions. Among beta-lactam antibiotics, piperacillin was more frequently synergistic than cefoperazone, cefazolin, or carbenicillin when tested in combination with each antineoplastic agent against various species.