Data for the study were collected over a 2-year period from February 1993 to January 1995. The data collected were regular measurements of the price and purity of street-level heroin; the date, location, and amount seized for all heroin seizures in Australia in excess of one kilogram; the number of persons admitted to methadone treatment programs in the study area; the availability of heroin as assessed by persons attending methadone clinics; a measure of local law enforcement; and the reasons for stopping use of heroin cited by persons seeking admission to methadone treatment programs. The study found that street-level law enforcement may be a factor in the rate at which heroin users seek treatment, but the current results cannot be viewed as indicating that more active street-level enforcement would increase the rate at which users seek treatment. Although variations in the quantity of heroin seized exert no impact on the street-level price of heroin, the risks created to heroin importers and distributors by supply-side law enforcement are probably determining factors in the high price of heroin. If the demand for heroin can be assumed to be sensitive to its price, it can be argued that the object of supply-side policy should be to maintain the price of heroin on the illegal market rather than to maximize the quantity of heroin seized. If the goal of supply- side law enforcement policy is to be to maintain the price of heroin on the illegal market, some means should be found for reducing the social cost of keeping illegally obtained heroin expensive. An expansion of the methadone program and the provision of heroin to dependent users under controlled conditions provide the best available means of reducing the social costs associated with supply-side law enforcement policy. Finally, the proposed ACT heroin trial should be used as a means of gauging the relative costs and benefits associated with the provision of methadone and heroin under controlled conditions to dependent users. 36 references and appended tabular and graphic data