Interviews with 40 instructors who had attended mastery learning workshops were conducted by the Chicago City Colleges to determine whether and why the instructors had modified and/or discontinued the mastery learning strategies and to correlate the degree of mastery learning implementation with teacher characteristics and attitudes. The instructors were asked about: (1) their college or high school teaching experience; (2) their attitudes toward 28 statements reflecting various teaching and learning principles; (3) the degree to which teachers actually implemented the four components of mastery learning (i.e., planning course objectives and a summative evaluation, designing a formative evaluation, planning corrective learning procedures, and implementing mastery learning tactics); (4) the modifications made during class; (5) the complexity of mastery learning; (6) the advantages and disadvantages of mastery learning; (7) the helpfulness of mastery learning workshops and other support services; and (8) the degree to which mastery learning was seen as a positive agent in helping students "learn to learn." About 33% of the instructors had modified the standard strategies, generally because of the time required to prepare them. The study report examines the correlations between the implementation of mastery learning and instructor characteristics and attitudes. The survey instrument is included. (JP)