Gut bacteria can boost the efficacy of antibody-based cancer treatments, according to two papers in Science (2015, DOI: 10.1126/aad1329 and 10.1126/science.aac4255). If the work in mice holds true for humans, cancer patients receiving immunotherapies might one day receive fecal transplants or probiotics to increase the presence of cancer-fighting bacteria in their guts. In one study, a team of researchers led by Laurence Zitvogel of Gustave Roussy cancer center found that the efficacy of ipilimumab, a monoclonal antibody used to fight skin cancer, was better at fighting tumors in mice with strong Bacteroidales and Burkholderiales populations in their intestines. Mice containing high levels of Clostridiales bacteria in their gut responded poorly to the treatment. In the other study, the University of Chicago’s Thomas F. Gajewski and coworkers found that feeding mice Bifidobacterium improved the outcome of monoclonal antibody treatments for skin melanomas. Sorting out the detailed interactions between gut bacteria and cancer immunotherapies may one day help clinicians modify patients’ microbiomes to optimize cancer treatments.