The Mentor Teacher Internship Program (MTIP) provided first-year provisional teachers with an experienced mentor teacher in the context of a supportive, non-evaluative peer relationship. The pilot program, implemented by the New York City Board of Education and the United Federation of Teachers, focused on increasing teacher effectiveness and teacher retention rate. The 1986-87 program was funded by the New York State Education Department's Office of Demonstration Programs. The participants included 42 mentors (teachers with at least 5 years of teaching experience in the New York City Public School system) and 73 interns from 23 schools in 13 community school districts in Manhattan, Brooklyn, Queens, and the Bronx. The study involved interviews with and surveys of participants and fellow staff, participant logs, and on-site visits. Significant findings are reported in the areas of staff development, expectations and final impressions of participants and other staff, and student performance. As of June 30, 1987, the MTIP has been generally effective in meeting its objectives. It is recommended that: (1) a substitute recruitment program linked to the MTIP should be implemented; (2) teachers who substitute for mentors should be made an integral part of the mentor team; (3) mentor-intern confidentiality should be retained; (4) principals, mentors, and interns should be informed about the MTIP prior to the school year; (5) temporary per diem teachers should be included as interns; and (6) mentors should be provided with additional staff development support. (TJH)