The plasma samples were also analysed for the presence of antibodies to FeLV whole virus, to FeLV p45 (the non-glycosylated form of gp70 surface unit of the envelope glycoprotein), and to FOCMA. Anti-FeLV p45 and anti-FeLV whole virus antibodies were measured by ELISA as described [21, 25] , using 100 ng of p45/well and 100 ng of gradient purified FL-74 FeLV, respectively. Plasma was used at a dilution of 1:200 and antibody levels assessed by comparison with predefined control antisera [25] . Antibody to FOCMA was measured at week 0 and week 20 p.i., by indirect cell membrane immunofluorescence as described [2] . FL-74 cell culture medium was tested for the absence of FCV, FHV, FPV, FCoV, FIV, hemotropic mycoplasma and presence of FeLV by RT-PCR/PCR as described [9, 13, 23, 31, 39, 44, 45] . The culture was consistently free of the unwanted contaminants. The cat sera were titrated at 4-fold dilutions from 1:4 to 1:256. Samples showing a minimal titre of 1:4 were considered to be FOCMA positive. In addition, samples from week À3 and week 20 p.i. were examined for the presence of antibodies to FeLV gp70, p27 and p15(E) [26, 30] by Western blot analysis as described [28] .