Cowdria ruminantium-induced production of IFN-[gamma] was measured by ELISA on a weekly basis during the course of vaccination with killed organisms emulsified in ISA50. Upon challenge, all (3/3) vaccinated animals that gave the lowest IFN-[gamma] response died of peracute cowdriosis. On the other hand, only one of three animals showing high IFN-[gamma] responses to vaccination died, but with a delay of 4 days in comparison with naïve controls. Thus, there seems to be a threshold level of IFN-[gamma] below which the probability for vaccinated animals to survive a lethal challenge is very low. During challenge, a much lower, but still physiologically meaningful production of IFN-[gamma] was detected using the 24-hour whole blood assay on day 5 after infection in animals controlling the infection. In contrast, IFN-[gamma] production was absent or negligible in naive and vaccinated animals that died within 8-10 days after infection. Although these results need to be validated on a larger number of animals, they strongly suggest that IFN-[gamma] is a useful indicator of protective immunity in animals immunized with killed Cowdria.