Gentamicin in combination with cephalothin (Gent-Ceph) or with chloramphenicol (Gent-Chloro) was utilized in the treatment of 55 infections occurring in 49 cancer patients. Responses were obtained in 78% of the infections treated with Gent-Ceph and in 64% of those treated with Gent-Chloro. Pneumonia and septicemia were the most common infections in this study. Among the cases of penumonia, 64% responded to Gent-Ceph and 67% to Gent-Chloro. Among the cases of septicemia, 88% responded to Gent-Ceph and 50% to Gent-Chloro. All of the identified organisms producing infection were gram-negative bacilli. Of these, E. coli was the most common. All organisms were resistant to cephalothin in vitro, and only 41% of them were resistant to chloramphenicol. However, resistant organisms responded significantly better to the Gent-Ceph combination (p less than 0.025). Also, response to therapy among patients with severe neutropenia (less than 100 neutrophils/mm3) was better for those patients treated with Gent-Ceph (p = 0.07). The combination of gentamicin with cephalothin or with chloramphenicol did not increase the frequency of side effects expected from gentamicin alone. No significant hematological toxicity was seen among those patients treated with chloramphenicol. Gentamicin in combination with cephalothin or chloramphenicol is an effective and safe antibiotic combination against gram-negative bacilli infections occurring in cancer patients. The efficacy of Gent-Ceph in patients with severe neutropenia is particularly advantageous.