The present invention relates to compositions, methods, and kits for predicting a subject's response to a CAR T cell therapy, by analyzing the intestinal microbiome of the subject. The present disclosure also provides a method of detecting patients at risk for a poor response to CAR T cell therapy by measuring the level of the presently disclosed bacteria or bacterial genes in the microflora or microbiome of a patient receiving or considered for CAR T cell therapy. The present disclosure further provides therapeutic compositions and methods for treating a subject having a cancer, by improving the subject's response to a CAR T cell therapy.