Human immunodeficiency virus type 1 (HIV-1) replication is regulated by several extracellular signals. We demonstrate that intercellular adhesion molecule 3 (ICAM-3) acts as a costimulating molecule to increase HIV-1 transcription and viral production, a process allowing productive infection of quiescent CD4+ T lymphocytes. The present work suggests an important role for ICAM-3 in HIV-1 replication.