For over 4 years, our international collaborative research team has carried out a social network HIV prevention intervention trial with community populations of young men who have sex with men (YMSM), disadvantaged ethnic minority Roma (Gypsies), and high-risk heterosexual adult (YHA) women and men in Hungary, Bulgaria and Russia. Eastern Europe has seen a sharp increase in HIV incidence, and social network interventions are high in cultural relevance because post-communist populations have a long history of trusting and relying upon their personal networks more than their governments. Across the three countries represented in our research during the past funding period, we enrolled social networks of YMSM, Roma, and YHAs. This study will renew our international collaboration to extend this network intervention approach. Our study, to date, has worked with very small independent friendship groups (usually composed of 5-6 people) as "egocentric" social networks.