The human gut has many bacteria and other microscopic creatures that live inside us, but do not harm us, and may indeed contribute to our health. Now two studies suggest that these bacteria are required for a full response to cancer treatment. Mice that were raised in a sterile environment (and thus lacked any gut bacteria), or whose gut bacteria had been destroyed with antibiotics, were implanted with cancer cells. When these mice were treated with immunotherapy or chemotherapy drugs, their tumors shrank less than those of mice with intact gut bacterial populations. These findings raise concerns for cancer patients, who are frequently treated with antibiotics to control infections due to weakened immune systems.