The demographic characteristics of the typical volunteer, taken from the personnel files of 208 current and previous volunteers of a Big Brothers agency, indicate the following profile. The modal Big Brother is usually Anglo-American, is a young adult aged 18 to 25, is a student or a full-time employed person, has some college education, lives in areas of the city heavily populated by students but not Little Brothers, is single, has no children, and has lived in the community for between one and five years. A sample of 156 of these volunteers was used in the determination of demographic characteristics which discriminate more and less effective volunteers. Effectiveness was assessed as length of service. Analyses of variance indicated that occupational status had the strongest relationship to effectiveness. More effective volunteers tended to come from white-collar occupations; less effective volunteers came from blue-collar occupations, or student or military backgrounds. When nine volunteer characteristics were used as predictors of effectiveness in a multiple regression analysis, significant results were obtained. The four variables contributing most were: occupational status, educational level, marital status, and employment-student status. (Author)