The human T cell lymphotropic virus type I (HTLV-I) is transmitted by blood transfusions that contain cell components as one of the virus transmission modes. We carried out mass screening of sera from seropositive donors by a gelatin particle agglutination test to prevent the spread of HTLV-I by transfusion. Seroconversion rates were compared before and after screening. The results show a remarkable decrease in the rate of infection from 53.6 to 0.9%. Therefore, it is certain that screening by the agglutination method prevents the spread of HTLV-I transmission in blood transfusions.