The research was conducted between July and September 1993 in Denver, Houston, New York City, Flagstaff, Ariz., Hartford, Conn., Oakland, Calif., and San Juan, Puerto Rico. Ethnographers observed 54 injection episodes involving two or more injection drug users who were not in treatment. The researchers gained access to the drug users through the assistance of an individual known to the researcher or outreach worker to be actively engaged in the injection of illegal drugs. After the observations, the drug users were offered risk-reduction information, including HIV prevention counseling, testing, and referral services. Researchers then developed a typology of higher-risk, lower-risk and nonsharing-risk networks. Results were that multiperson use of injection paraphernalia or drug solutions occurred in 94 percent of the injection events. Serial use of syringes and needles occurred infrequently (14 percent) relative to backloading (37 percent) and reuse of paraphernalia (cookers, 84 percent; cotton, 77 percent; water, 77 percent). Findings also indicated that higher-risk drug injection networks were characterized by larger sizes and the pooling of resources for drugs. Findings indicated that prevention messages must include discussions of avoiding the reuse of injection paraphernalia and the transfer of drug solutions. Tables, figure, notes, author biography and photograph, and 51 references (Author abstract modified)