B. S. Bloom's theory of school learning states that three independent variables account for most of the variation in students' achievement: cognitive entry behaviors (CEB), affective entry characteristics, and the quality of instruction. These variables determine the nature of learning outcomes--level and type of achievement, rate of learning, and affective characteristics. Bloom developed his mastery learning strategy to produce successful learning through modifications to student entry characteristics and instructional behaviors. A study examined the interactive effect of CEB with mastery versus non-mastery learning strategies of instruction on reading comprehension. Subjects, 180 third grade students, were randomly assigned to four treatment groups--two mastery learning groups (90% and 70%) and two control groups (non-mastery and control). They were individually instructed in a sequence of three learning tasks that were designed to teach word recognition, word meaning, and word order. Several assessment instruments were used to measure the children's CEB and learning outcomes. Results showed a significant overall CEB and treatment interaction on immediate achievement in comprehension. Low CEB students benefited relatively more than high CEB students from the mastery learning treatment. The findings confirmed the interactive relationship between initial abilities and instructional methods postulated by Bloom. The CEB plus treatment interaction on long-term retention was not significant, suggesting the need for extended mastery learning programs for low CEB students. (FL)