Sera from a population of 212 farmers of the Puy de Dôme (Midlands of France) who are in close contact with Ixodes ricinus, the vector of Lyme disease in Europe, and sera from 100 urban blood donors from the nearly city of Clermont Ferrand were examined by microimmunofluorescence (MIF) for antibodies reacting with Borrelia burgdorferi. The results showed a higher seroprevalence of IgG > 1/100 in farmers (25%) than in blood donors (10%). Using western blot with antibody at a 1/200 dilution, and regarding sera as being positive if they contained IgG reactive with more than 5 bands, 93/212 farmers (44%) and 20% of blood donors were positive. Reactions with specific protein bands (94, 73, 30 and 21 KDa) by western blot were demonstrated in 83/212 sera from farmers (39%) and 16% of blood donors. Both methods showed a higher seroprevalence in the farmer population. Western blot is a sensitive and specific test for seroepidemiology but, in highly endemic areas, it is not diagnostic for active Lyme borrelliosis.