The process of acquiring, preparing and dividing up injectable drugs entails an array of behaviors that expose injection drug users (IDUs) to HIV and other blood-borne pathogen. The behaviors include the common use of drug preparation materials and the use of a single syringe to mix, divide and distribute jointly purchased drugs. These risk behaviors can take place even when IDUs are not injecting with the same syringe (i.e., needle sharing). There is compelling evidence that drug preparation (DP) risk behaviors are associated with HIV and hepatitis C virus (HCV) transmission. However, not much is known about the determinants of DP behaviors and very few intervention strategies have been designed to explicitly help IDUs reduce this risky practices. We propose to conduct formative research to identify the determinants of DP risk behaviors among IDUs in Puerto Rico. The goal of the proposed project is to generate the knowledge-base with which to develop an R01 proposal to test the efficacy of a preventive intervention aimed at reducing DP risk behaviors. The proposed formative research has been designed to be implemented in three phases. In a Qualitative Phase we will conduct participant observations, focus groups, and semi-structured interviews to describe in detail the DP behaviors practiced by IDUs in Puerto Rico, and the circumstances under which these behaviors are practiced. In the second phase we will use the findings of the Qualitative Phase, combined with theoretical constructs from several major behavioral theories to develop measures of the DP behaviors and of the factors that can potentially account for these behaviors. In this Measurement Development Phase, we will develop and pilot test a structured interview protocol to measure DP behaviors, as well as the candidate explanatory factors. Finally, we will conduct a prospective study. In this study we will administer the interview protocol to a sample of 400 IDUs. Two weeks after baseline assessment, participants will be interviewed again. The analytical plan will assess the ability of the developed variables to account for DP risk behaviors and will help to identify the prediction of these practices.