This paper deals with measurement of language dominance at the early-childhood level using a rating scale to help bilingual programs with student classification and placement. Some of the assumptions unique in the measurement of language dominance are discussed and applied to the validation procedure on a Spanish/English language dominance scale which is under drastic revision. The instrument was developed for children whose home language may be Spanish, and is designed to be individually administered by a bilingual tester. Part 1 is administered in Spanish and Part 2 in English. The screening discriminates among children in three categories; those preferring English, those preferring Spanish, and bilingual children. The instrument was given to 30 four-year-old Mexican-American children attending three day care centers in the summer of 1974. Teachers' judgments and test results were compared to determine validity, resulting in a coefficient of .86. The test-retest and the rater's reliability coefficients were quite high, which may indicate that the test manual was clearly written, the categories were well defined, and the children performed consistently on this variable. It may be concluded that this is a reliable instrument for four-year-old Mexican-American children at the testing site. (RC)