Serologic studies were performed to determine if BKV and JCV are transmitted congenitally and to assess if the virus present in donor kidney contributes to virus activity in renal transplant recipients. Fourteen of 100 normal women showed antibody rise to BKV or JCV during pregnancy; in all instances, these were reactivation infections occurring in initially seropositive women. BKV-and JCV-specific antibodies of the IgM class were not detected in over 300 umbilical cord sera. Thus, there was no evidence of congenital transmission of BKV or JCV. BKV-seronegative renal transplant recipients who received kidneys from BKV-seropositive donors had a frequency of BKV infection which was about four times greater than that in BKV-seronegative recipients receiving kidneys from BKV-seronegative donors. These data suggest that BKV in donor kidney contributes to primary BKV infections in renal transplant recipients.