Findings are presented from studies on the use of radio for teaching primary school children mathematics in Honduras and Bolivia and English as a Second Language in Lesotho. Interactive radio instruction (IRI) is so called because of the active participation of the students. Although lessons are presented by conventional radio, scripts are written to solicit responses from the children. Data from Honduras and Bolivia include both costs and measures of effectiveness, but data from Lesotho are for costs only. In Honduras, almost 200,000 children and 6,000 teachers used the radio lessons in 1990. Evaluations were conducted with three groups from different parts of the country. In Bolivia, where lessons reach about 50,000 children, evaluations have been conducted for grades 2, 3, and 4. English language instruction in Lesotho is delivered by radio to about 200,000 children; in contrast to Bolivia and Honduras, broadcasts are on government-owned, rather than privately owned, radio stations. Data suggest that IRI is a powerful instructional tool, possibly even more cost effective than textbooks or teacher training. Achievement gains in Bolivia and Honduras were striking. IRI should be considered along with textbooks and teacher training when educational planners consider options for improving basic education. Twelve tables and eight figures present study data. (Contains 16 references.) (SLD)