Calves were immunised by means of a native Al(OH)3-absorbed gp51-preparation which had been obtained from a foetal lamb kidney cell line, following bovine leucosis virus (BLV) infection. 9 animals were immunised 3 times, using 300 micrograms gp51. 7 animals underwent test infection, using 2.5 x 10(3) or 2.5 x 10(4) BLV-infected lymphocytes. Serological and virological reactions of all animals, including 3 calves which had received only test infections, were followed up through 40 weeks by means of immunodiffusion test, enzyme-immuno-assay, gp51 antibody radio-immuno-assay, reverse transcriptase test, syncytial test, competitive p24 radio-immuno-assay, and by transmission of whole blood in animal experiments. The results obtained from virological testing showed that 1 animal had been protected by preceding immunisation. 4 in 7 immunised and test-infected animals exhibited transient BLV values, between the 7th and 16th weeks from infection. Typical leucosis infection had been induced to 2 animals. The above findings are discussed and are compared to similar results recorded by other working groups.