This paper examines the selection and influence of friends in schools which differ in the degree of "openness" in the instructional program. Data collected from over 4,000 students in grades 5, 6, 8, and 11 in 1973 and 1974 provide information on student characteristics, school structure and processes, and academic and affective outcomes for students and their friends. Results show that school structure affects the selection of friends in grades 6-12. In the more open schools friendships are less structured. Fewer students are isolated, more friendships are reciprocated, and friends are selected from a wider set of contacts. Also, students who score low on selected measures are included in friendship groups and more often have high scoring students as friends. In studying the influence of early friends on later behavior, results show that at all grade levels high scoring friends are an important factor. Students with initially low or initially high scores can be positively influenced academically and nonacademically by friends with high scores. A final analysis examines unreciprocated, reciprocated, and stable friendships, the improvement of measures for research on peer influence, the appropriateness of current theories of friendship selection and influence, and the implications for school practice. (Author/KC)