By measuring antibodies against FeLV p45 and whole virus by ELISA, seroconversion was observed in one cat. In the p45 ELISA, this cat (JCR2) had a positive response throughout weeks 8 to 20 with a peak of 26% of the positive control at week 14, and throughout weeks 4 to 20 with a peak of 54% of the positive control at week 20 in the FeLV whole virus ELISA (data not shown). Analysis of the remaining animals showed no statistically relevant difference between the groups in p45 as well as whole virus ELISA (P MANOVA 0.53 and 0.47, respectively Figs. 2 and 3). Prior to challenge, all samples were negative for the presence of antibodies directed against FOCMA. At week 20 p.i., antibodies to FOCMA were detected in 4 cats (KCT1, JCU3, JCO2, JCP1, 57%) of group 10K and 4 (KDA1, JCR2, KCQ1, JCS1, 57%) cats of group 100K. Of these 8 cats, the PCR-positive cat (JCR2) reached the highest titre (1:64), the other 7 cats developed antibody titres which ranged from 1:4 to 1:16. No negative control group and group 1K animal had a detectable titre (Tab. I). In Western blot analysis, sera from 2 cats of group 10K (JCU3, JCS2) showed a weak reactivity to the upper band of the p15(E) protein at a serum dilution of 1:40 as well as at 1:100. However, the same cats showed some reactivity to the upper band of p15(E) already at week 0 p.i. (Fig. 4) . In the PCR-positive cat (JCR2) the presence of antibodies against FeLV was confirmed at a dilution of 1:100. All other cats from group 100K, 10K and 1K had either a completely negative Western blot pattern or showed cross-reactive bands already present at week À3 (cats KDA1 and JCS2, Fig. 4 ).