This protocol describes methods for a mouse xenograft study designed to measure in vivo BiTE\u00ae\u00a0efficacy against antigen-expressing human tumor cells. In a modified version, the protocol is used to measure BiTE\u00ae-mediated bystander killing. Mixtures of human EGFR-positive (unlabeled) and EGFR-negative (luciferase-labeled) cells are implanted in nude mice along with human T cells. Mice are treated daily with BiTE\u00ae, beginning one\u00a0day after tumor implant\u00a0for 20 days. Growth of EGFR-negative cells is measured by imaging (luminescence) and by calipers (tumor volume).