This research report recounts the procedures, results, and recommendations of a research project in which more than 200 tenth-grade students in Florida were tested (1) to determine whether aptitude treatment interaction (ATI) effects on syntactic maturity and on knowledge of structural relationships occur after several months of instruction; (2) to refine the ability measures used as predictors in this study to increase their differential validity in measuring ATI; (3) to determine whether the findings from the first study could be cross-validated in a second study. The following topics are discussed: (1) previous studies of ATI effects; (2) the complexities involved in any research which attempts to enhance student achievement by using instructional treatments related to ability patterns; (3) the criterion measures used to pretest the students' general, semantic and symbolic abilities and to pre- and post-test their grammar achievement; (4) the effectiveness of the two linear-programed textbooks in transformational and traditional grammar which were used as treatments (5-month period for the first study and 3 months for the second); (5) cross-validation procedures; and (6) the results of the research which, though inconclusive, pointed to modifications in treatments, scoring techniques, and ability tests . (JB)