As part of an educational reform in the Netherlands, teachers were required to implement models of adaptive instruction, including group-based mastery learning. It was expected that mastery learning would raise achievement and reduce differences in achievement among children of different social classes and between girls and boys. In 1987, students and teachers in grades 5 and 7 in 220 elementary schools participated in a study of the performance of the model. In 1988, the same students (who were then in grades 6 and 8) and their teachers participated. In all, about 10,000 children and 1,000 teachers were included in the study. All data were collected using questionnaires and Dutch standardized tests. Mastery learning was very popular for teaching mathematics, but the Dutch language was generally taught in a conventional way. In comparison with conventional instruction, few effects of mastery learning were found. Positive effects were found for language achievement in grade 5, and negative effects were found in the language achievement gain of grade 5 to 6. Differences in the achievement of children of different social classes and of boys and girls were not reduced by mastery learning. When the effects of other forms of adaptive learning (ability grouping and individualized instruction) were studied, mastery learning was the only form that did not lead to lower achievement scores than did class instruction. Possible explanations for the lack of success are reviewed. Eight tables present study data. (SLD)