The design and effectiveness of the Amigos program, a two-way Spanish-English bilingual immersion program in Cambridge (Massachusetts) are described. In the program, half the instruction is in English, half in Spanish from kindergarten through eighth grade. Half the students are native Spanish-speakers and half are native English-speakers. The report begins with a description of the program's design, then presents two brief case studies of two native Spanish-speaking program graduates, including comments from interviews. The bulk of the report consists of the results of an attitude survey administered to all students in 1996 and of standardized tests of English and Spanish skills, including reading and mathematics subscales from 1990-91 through 1996-97. It is concluded that both native English-speaking and native Spanish-speaking groups are approaching balanced skills in the two languages in reading and math, and are gaining appreciation and knowledge of the others' cultures. Contains 22 references. (MSE)