Cell-free and cell-associated human immunodeficiency virus cultures suspended in 10% serum remained infectious for several weeks at room temperature. The stability was further increased when cell-associated virus was suspended in neat serum. When dried onto a glass coverslip, virus remained infectious for several days, although cell-associated virus lost infectivity more rapidly than cell-free virus. Many parameters influence the capacity of a virus to survive in the environment, including the concentration of virus, the temperature, the humidity, and the nature of the surrounding medium. The rate at which infectivity is lost is an important determinant of the potential risk of transmission.