Plague is a re-emerging disease endemic in at least 24 countries. Non-endemic countries should
be able to confirm plague to prevent outbreaks due to imported cases. We established a
combination of a IgG/IgM screening ELISA and a confirmation immunoblot employing F1
capsular antigen (CA) for the serodiagnosis of plague in countries where yersiniosis is present.
The ELISA and the immunoblot assay showed a specificity of 96·1% and 100% among sera
from healthy German blood donors. This group had a seroprevalence of 39% of anti-yersinia
outer protein (YOP) antibodies obviously caused by previous Y. enterocolitica infection. The
ELISA detected anti-F1 CA antibodies in 22 and the immunoblot in 20 out of 26 sera of
plague vaccinees. Five control sera from bacteriologically confirmed plague cases from
Madagascar reacted positively. It can be concluded that anti-YOP antibodies do not affect
assays based on purified F1 CA.