Little information is available on the immune response to parvovirus B19 after the administration of contaminated blood products. In the present study, we found that levels of B19 IgG in B19-seropositive recipients protect against reinfection and, after transfusion with pooled plasma containing B19 DNA (1.6 x 10(8) IU/mL), increase from 19-39 IU/mL to 50-100 IU/mL. We found that, in the presence of 1.6-2.2 x 10(8) IU of B19 DNA/mL in B19-seronegative recipients, a pooled-plasma B19 IgG level of 59.5 IU/mL is insufficient to prevent B19 transmission and subsequent seroconversion. These data should lead to improvements in the assessment of blood-product safety. These data should lead to improvements in the assessment of blood-product safety.