The study focused on the influence of peer group counselling and school influence on the self-concept of adolescents' in Nigerian secondary schools. Sixty-eight Senior Secondary School II students from three schools--a boys' school, a girls' school and a co-educational school in Benin City participated in the study. A pre-test, post-test control group experimental design was employed for the study. A 40-item self-concept scale adapted from Akinboye (1977) APDI, was validated and used to measure the self-concept of the participants. The internal reliability for the scale is reported to be 0.70 (Cronbach's alpha), and the test-retest reliability was 0.87. Three hypotheses were formulated and tested at 0.05 level of significance. Results of the statistical analysis of data indicated that peer group counselling had a significant positive effect on the self-concept of the participants (t-value of 6.58 is greater than the critical t-value of 1.96, df, 66). Similarly, school type was found to have a significant effect on the self-concept of adolescents--girls in the single-sex school had significantly higher self-concept than girls in the co-educational school. (F-value of 5.024 with df (3, 52). A significant interactive effect on self-concept was found between the treatment and school type. This interaction was ordinal and did not qualify the main effects. Implications for counselling are discussed. (Contains 5 tables.)