As an alternative approach to individually-focused understanding of HIV risk behavior, this study explored the relationship between social network characteristics and HIV-risk injecting behaviors. Subjects were 499 inner-city injection drug users (IDUs) recruited from the streets of Baltimore, Maryland. Analysis of structural and functional network data indicates that a substantial proportion of drug sharing network members also provided social support, often because of family and sexual partner relationships. IDUs with larger drug networks which also provided social support were more likely to share needles, while IDUs with larger drug networks which did not provide social support were more likely to inject in commercial settings. The findings suggest that social support from drug network members has differential effects on injecting HIV-risk behaviors among IDUs. HIV prevention efforts should be expanded to reach drug-sharing networks and should take into account their social support relationships.