In this paper, we describe an application of responsive virtual humans to train law enforcement personnel in dealing with subjects that present symptoms of serious mental illness. JUST-TALK provides a computerized virtual person to interact with the student in a role-playing environment. Students were able to converse with the virtual person using spoken natural language and see and hear the virtual human s responses a combination of facial gesture, body movements, and spoken language. The JUST-TALK project, funded by the National Institute of Justice Office of Science and Technology and developed by RTI International, involved integrating virtual reality training software within a 3-day class at the North Carolina Justice Academy. The course was structured to include classroom-based lecture, videos, discussion, live human role-playing, and virtual human role-playing. A scientific evaluation of the class and the software system was carried out by North Carolina State University. This assessment investigated the contribution of natural language interfaces and virtual reality technology to learning in this applied setting. Results of the evaluation are extremely encouraging. The vast majority of students (88 percent) found the simulation easy to use. A majority of the students said the virtual trainer enhanced their learning in the course. As a training tool, students rated the computer simulation on par with other training methods including lecture, role-play and discussion. A total of 59 percent of students felt the simulation was better for learning or comparable to role-play; 77 percent felt simulation was better than or comparable to lecture; and 59 percent felt the simulation was better than or comparable to discussion.