Sheep can be immunised against toxoplasma abortion with a live vaccine composed of S48 tachyzoites. In this study the immunity induced in sheep was examined 18 months after vaccination. Seventy-three sheep were divided into three groups. Group 1 was vaccinated and 18 months later, when they were 90 days pregnant, the 27 sheep were challenged orally with Toxoplasma gondii oocysts. These ewes produced significantly more live lambs, after a significantly longer gestation period, than the 34 unvaccinated, challenged ewes in group 2. In addition there was less placental damage and fewer lambs were born with antibody to the parasite in the vaccinated group than in group 2. The 12 ewes in group 3 served as unvaccinated, unchallenged control animals. The degree of protection induced by the S48 tachyzoites was as good 18 months after vaccination as previously observed six months after vaccination.