Participant observers, working openly or covertly, are often able to penetrate the world of illicit drug use and explain the behavior of drug users. They are able to obtain urgently needed qualitative and quantitative data, but at the same time, they face ethical and practical problems that can compromise their own and their clients' interests. The author argues that while the participant observer must remain loyal to the subjects of the study, he is obliged to intervene when serious threats to the safety of the subjects occur. AIDS is a serious health threat to illicit drug users. Participant observers should be trained to dispense health education information to drug users in order to reduce the possibility of AIDS transmission. 39 references. (Author abstract modified)