The Team-Assisted Individualization (TAI) mathematics program has been developed in an attempt to make individualized instruction workable in the classroom by adding components of cooperative learning. This paper presents the rationale for the development of TAI and describes results of three field experiments conducted to assess the effect of TAI on student achievement, attitudes, and behavior. The experiments, which involved a total of 1,997 students in grades 3 through 6 (including mainstreamed students), demonstrated basic achievement effects of the program and a number of positive social and attitudinal effects. Results clearly indicated that TAI increases students' mathematics achievement more than traditional instructional methods and that TAI students gained more than their control counterparts on every achievement measure in every study, although the differences were not statistically significant on some subscales at some grade levels in the third experiment. In addition, the third experiment demonstrated that TAI could be used over an extended time period (most of a school year) as the primary means of mathematics instruction. Among the other results reported are those indicating that TAI can have a strong positive effect on the social acceptance and behavior of academically handicapped students. (JN)