Injection drug users (IDUs) in San Juan, Puerto Rico are characterized by high rates of daily injecting, injection of shared drugs, re-use of injection syringes, and use of shooting galleries. They lack adequate access to new injection syringes and drug preparation equipment, and experience elevated rates of HIV and HCV infection. Between April and August, 2006, researchers and active IDUs collaborated in the development of an experimental HIV/HCV intervention aimed at identifying drug preparation items and practices that will enable IDUs to make drug solutions without potentially contaminated injection syringes contacting materials used to prepare drugs. The collaboration involved discussing and testing a variety of drug preparation items and practices in office and community settings. The process was repeated until concerns that had been raised were resolved, and a tentative set of intervention items and practices to be evaluated in a community field trial was identified. Throughout, a strong emphasis was placed on the capacity of an item or practice to address common problems confronted by IDUs (blunted needles, clogged syringes, injected particles) in addition to the core aim of reducing contamination of preparation materials by blood in injection syringes. This report describes the final selection of items and practices: 1) A small water bottle that permits IDUs to add approximately .05 cc water drops directly to drug powder in cookers; 2) A preparation syringe (a type of ancillary equipment not used for injecting) that permits IDUs to pull up a measurable amount of water to add to drug powder, an alternative to producing water drops; 3) A filtering device, the Sterifilt filter, attached to a preparation syringe, which eliminates the need for cotton or cigarette filters; 4) Use of a preparation syringe to distribute drug solution by backloading to injection syringe(s); 5) A small water bottle enabling IDUs to clean injection syringes by backload rinsing. The overarching aim of this experimental HIV/HCV intervention was to promote the safe re-use of drug preparation and injection items, and to impact the large number of IDUs in San Juan who maintain personal injection syringes, but currently use communal ancillary equipment in shooting galleries and inject drug solutions prepared with other IDUs' injection syringes.