The antibacterial activity of the combination of carbenicillin with cephalothin, in a ratio of 4: 1, was studiedin vitroand in volunteers. It was shown that a mixture of 100µg/ml of carbenicillin with 26 µg/ml of cephalothin inhibited 97 per cent of 535 gram-negative rods isolated from patients in a cancer hospital. Sera obtained from patients receiving the combination were inhibitory and bactericidal at a higher dilution than sera obtained from the same patients receiving carbenicillin or cephalothin alone (the geometric means were 1/16 and 1/8, respectively). It is concluded that carbenicillin plus cephalothin might prove to be effective as an empiric therapy for suspected gram-negative sepsis.