Sera from 94 healthy Finnish and 100 healthy German blood donors were studied for Yersinia enterocolitica antibodies by two different techniques, enzyme immunoassay and immunoblotting, to compare the frequency of Yersinia infections in Finnish and German populations. The prevalence of Yersinia antibodies in Finland (19% and 31% by enzyme immunoassay and immunoblotting, respectively) and in Germany (33%, 43%) is relatively high and may indicate many subclinical Yersinia infections in healthy populations. Postinfectious complications, such as reactive arthritis, may follow also such subclinical infection; sensitive serological tests are important in their diagnosis.