Members of the National Center for Research in Mathematical Sciences Education Working Group on the Teaching/Learning of Quantities began research in 1991 to identify the teacher characteristics associated with individuals considered by their peers as leaders or early adopters--whose mathematics teaching already exemplifies the spirit of the (NCTM) National Council of Teachers of Mathematics Standards. This article describes findings from a preliminary study which sought information on middle school mathematics teachers' conceptions of teaching, their pedagogical and content knowledge, and their teaching behaviors. Data were obtained on teacher characteristics by conducting extensive interviews, an assessment of content knowledge, and observations during a series of 3-hour seminars over a 3-month period. A framework containing five categories of conceptions of mathematics teaching was used. The five categories are: what mathematics is, what it means to learn mathematics, what it means to teach mathematics, what the roles of the teacher and the students should be, and what constitutes evidence of student knowledge and criteria for judging correctness, accuracy, or acceptability of mathematical results and conclusions. Findings revealed that these teachers: focused on problem solving, conceptual relationships and understanding, and communication in mathematics; had a comprehensive knowledge of the mathematics they were teaching; and participated in their own professional growth by attending conferences and inservice programs, completing graduate studies, and continuing to seek encouragement and support for their reform efforts. (MKR)