An instrument was developed to assess principal leadership. Two studies were then conducted to assess the reliability, validity, and utility of the instrument. Leadership style is the relative intensity of the presence of four modes of authority (traditional, charismatic, legal, and expert authority) and four modes of power (moral, psychological, contractual, and technical power) in leader action. The developed instrument--the "Principal Leadership Profile" (PLP)--contains 34 assessment items and 14 demographic questions. In the initial study--a test of assessor reliability and construct validity--experienced and novice principals (from elementary, middle/junior high, and high schools) were assessed by six teams, each with a teacher, a principal, and a central office administrator. In all, 12 elementary, 9 middle, and 12 high school principals participated; and 6 teachers, 7 principals, and 5 central office administrators evaluated them. A second study, of interrater reliability, used 4 videotaped candidates and 28 volunteer assessors, all educators. Assessor reliability was relatively high, as was construct validity. A moderate level of interrater reliability was found. With further development, the instrument holds promise for assessing the leadership of school principals. Fifteen figures and five data tables conclude the document. The PLP and the questionnaire for assessors are appended. (SLD)