On the basis of interviews with addicts, experimenters, and nonusers, 11 macroepidemics were identified, along with 28 microepidemics. Upon further study, it was found that the majority of addicts were produced by macroepidemics which occurred in a relatively small number of neighborhoods. The distinctive features of the process of heroin spread were: it tended to be most "contagious" during the early stages and was spread by new users and by the newly addicted; it was spread through person-to-person contact and the individuals involved tended to be close friends or relatives; and the maintenance of a heroin habit brings new addicts into frequent association with those in the neighborhood that they may not have known prior to their addiction. Events that might halt heroin spread are removal of one or more key members of the distribution structure in a microepidemic. In macroepidemic neighborhoods, however, this would have little effect because heroin is available from multiple sources. 4 figures and 6 notes.