The extensive use of vaccination should be supported by some knowledge on the protection afforded, the possibilities of use in adult sheep and its effects on immunological tests. In this paper we will present preliminary results on some of those aspects, obtained in a field study with an attenuated vaccine widely using in Spain. The study has been carried out for more than two years on six sheep flocks in which annual incidence of clinical cases ranged between 0.5% and 6%. The vaccine was applied to about a half of the replacer ewe lambs of each flock at about one month of age. Moreover, half of the adult ewes were also vaccinated with the same dose. Before vaccination and about six months afterwards, immunodiffusion (ID) and ELISA were carried out on the replacers. Once per year all ewes older than a year were submitted to ELISA and ID. Up to now only two non-vaccinated ewes from the control group have appeared with clinical signs of PTBC. There was a significant correlation between the results of ID and ELISA of the lamb before vaccination and those of its dam. There was also a correlation between results at birth and at six months, both in vaccinated and in non-vaccinated lambs. The first group of vaccinated replacer ewes showed a decrease in the frequency of positives in the ID test from 48.2%, about 13 months after vaccination, to 13.2% at 23 months post-vaccination. The group vaccinated in the following year had 37.2% reactors by the 6th month and 26.9% by the 9th month. The control groups changed from 0.0% to 1.4% and 3.2%, respectively. The first group of ewes vaccinated as adults were 15.0% positive by the 18th month post-vaccination, while the second one were 9.9% positive by the 6th. In the ELISA, all the groups appeared to keep high levels of reactivity (about 90%) at all the times. It could be pointed out that transfer of humoral immunity by colostrum seems to occur, which apparently does not interfere with response to vaccination. The relatively fast decline in the percentage of positives in the ID test after vaccination, contrasts with the maintenance of high levels in the ELISA.