A study was undertaken to determine whether four commercially prepared language programs would bring about better gains in the language achievement of disadvantaged preschool children than would activities planned by teachers themselves. The four commercial programs included the Peabody Language Development Kit: Preschool (PLDK-P); the Ginn Language Development Program-A (GLDP-A); the Open Court Language Development Program: Kindergarten (OLLDP-K); and the Distar Language Program: I (DLP-I). In addition, a program called "Control Language Development: Kindergarten" (CLDP-K) was used; this consisted of activities planned by the teachers themselves that did not follow a prescribed manual or outline. Children in 10 Head Start classes were randomly assigned to a treatment group consisting of two teachers and their classes. All 140 children met the criteria for "disadvantagedness" based on parent occupation and other indices on the Office of Economic Opportunities poverty guidelines. Subjects were tested once prior to the experiment for their initial language skills and then again after 60 days of instruction. Comparisons of subjects' pretest and posttest scores indicated that all four structured programs produced better gains than did the teacher-made materials. Of the four programs studied, the Peabody Language Development Kit (PLDK-P) seemed to produce better general results than did the others. (MP)