Two experiments were conducted to investigate the use in mainstreamed classes of team assisted individualization (TAI), in which students work on individualized units in heterogeneous, cooperative learning groups. Team reward systems are incorporated into the small group instruction format. The effects of the TAI approach on the achievement, attitudes, and behaviors of students in general and the behavior and peer acceptance of mainstreamed academically handicapped (MAH) students were measured over 8 weeks for 504 students in grades 3-5 in experiment 1; experiment 2 assessed the mathematics achievement effects of TAI on MAH and non-handicapped students in grades 3-5 (n=1,371) over a 24-week period. Results of experiment 2 confirmed the hypothesis that achievement effects were not seen for the MAH subsample in experiment 1 because of the brief duration of the study. Over the full 24-week experiment, MAH Ss in TAI classes gained much more on the mathematics scales than did their control group counterparts. In both studies, nonhandicappped Ss also gained markedly in mathematics achievement in the TAI classes as compared to control Ss. Results were also positive for the behavior and social acceptance of MAH Ss. Sociometric results indicated that when MAH Ss work in small groups with nonhandicapped classmates, they are better accepted than Ss not working in such groups. Implications of the TAI approach for mainstreaming were discussed. (CL)