The spread of HIV by blood transfusion represents a highly efficient mode of transmission. Blood components from donors with antibody to HIV have been shown to transmit the virus with frequencies of greater than 90 percent. Packed red blood cells, platelets, or fresh frozen plasma are all equally effective in transmitting the virus to recipients. It is highly likely, therefore, that recipients who receive blood from donors with antibody to HIV will themselves become infected with the virus. Fortunately, the likelihood of such an occurrence in the United States is now rare due to the implementation of screening procedures and other safeguards. When the study was initiated in 1989, there was a growing concern, however, that human retroviruses other than HIV might threaten the safety of the United States blood supply.