The COPE efficacy study compared the NIDA standard intervention against two culturally targeted, enhanced interventions, one for African Americans and one for Puerto Ricans. The study compared a sample of 188 out-of-treatment injection drug users with matched baseline and 6-month follow-up interviews for changes in monthly injection rates and proportionate use of new and preused needles and used injection supplies. Effects of injection outliers, attrition, and ethnic differences were examined for impact on outcome measures and to identify subgroups within the study population for whom intervention had differential effects. Preliminary analysis suggested attendance in culturally targeted, enhanced interventions may increase the likelihood of positive program outcomes, including drug-related risk reduction for some populations. However, subgroups of injection drug users, such as extremely high injectors or individuals who drop out before completing the program, may require different intervention approaches. Further research is recommended to understand the relationship between intervention and behavior change, reasons for attrition, and moderating factors affecting project outcomes. 59 references, 6 notes, and 7 tables