Considering that the assay for proviral DNA is able to detect one copy per reaction, we believe that the consistent PCR-negativity in blood of the cats that seroconverted is a strong indicator for the absence of bone marrow infection, a confirmation that an alternative course of infection may indeed exist, where the bone marrow is not involved [1] . FeLV provirus may still be integrated into the genome of the cells of some tissues but, in most cases, at levels beyond detection. In the study presented here, FeLV DNA sequences were not detectable in the organs tested. However, the presence of FeLV provirus in tissues other than the ones we analysed can not be excluded and should be investigated further. In localized infection where FeLV replication is confined to certain tissues, presence of plasma viral RNA loads is regarded a sensitive parameter for infection. RNA loads reflect ongoing viral replication somewhere in the cat's body even in sequestered places. However, detection of RNA is generally less frequent than that of proviral DNA [39] . Seroconversion was observed by use of the FL-74 cell membrane immunofluorescence test. This assay originally had been designated FO-CMA test and was thought to detect antibodies at a non-virion tumour-associated antigen [2] . Later, it became clear that the FOCMA phenomenon can be explained not by a tumour specific antigen but by viral proteins including FeLV p15(E), gag-polyproteins and gp70 of FeLV subtype C present on the FL-74 cell membrane [43] .