This study explores several research questions. Of primary concern was the identification of variables that might be employed to predict the potential of students to succeed in monolingual English classrooms following transition from bilingual programs. Although consideration was given to various background and demographic factors, the principal focus of the research was on language proficiency variables--specifically reading comprehension, verbal production, and aural comprehension in English and the native language, Spanish. Of secondary and collateral concern was the validity and reliability of available instruments that might be used to measure these various language proficiencies. Two batteries of tests were constructed and refined. The appendix provides a list of the objectives or skills that were specified for construction of the batteries. A third concern was the comparability of results between different Hispanic language groups--students of predominantly native Mexican and Mexican American origin in the Los Angeles area and students of predominantly Puerto Rican and Dominican origin from the East Coast of the U.S. Analysis explores the relationship between language proficiency and classroom performance prior to and following transition for the current year's third-grade students enrolled in transitional bilingual programs, and the students who had been enrolled in third grade transitional bilingual programs the previous year. (Author/JK)