Human herpes virus 8 (HHV8) may be sexually transmitted, but transmission via blood cells has not yet been excluded. We used a modified immunofluorescence assay to detect Ab to HHV8 latency-associated nuclear Ag in sera of 200 allogeneic BMT recipients and their related donors. In control subjects, Ab were found in 85% of patients with AIDS-related Kaposi sarcoma (n = 52), 34% of HIV-1 infected subjects without Kaposi sarcoma (n = 56) and 9. 5% of blood donors (n = 42). Among BMT donors, 14.5% were HHV8+, while 10% of recipients were positive before, and 18% after BMT. In the 176 HHV8-negative recipients at BMT, there was no relationship between post-BMT seroconversion, which occurred in 26 cases (15%), and the donor's serological status. Of note, 10 HHV8+ recipients before BMT became negative post-BMT. Outcome of BMT was not influenced by prior HHV8 seropositivity, seroconversion or seroreversion of recipients. That HHV8 seropositivity among blood donors from the Paris area was comparable to that of BMT donors and recipients before BMT indicates that these patients had not been at risk of HHV8 by blood products received before BMT, although post-BMT HHV8 seroconversion probably corresponded to contamination by blood transfusions rather than by the BMT.