The study found that most heroin dealers operated independently, working either for themselves or in loosely defined groups of two or three, with little or no hierarchy. Other dealers acted as "runners," who sold heroin for other dealers for a percentage of sales. The dealers used a range of "folk" (unscientific) methods for testing the quality of drugs they purchased for sale; adulteration or "cutting" the drugs was rare, even during periods when heroin was scarce. In contrast to the popular perception that heroin dealers are highly competitive with one another to the point of violent attacks on competitors, this study found that dealers in both areas cooperated with one another; little intimidation or rivalry was reported or observed. The dealers reported violence to be rare and largely avoidable. The findings were obtained from interviews with 32 street-level heroin dealers. They were asked about their experiences in selling drugs, the organization of the heroin market, drug adulteration and quality-assessment practices, and the extent and impact of violence linked to the operation of heroin markets. 1 table and 65 references