In the autumn of 1989 a seroepidemiological study was carried out among 440 owners of hunting dogs and 1052 blood donors from different regions in the Netherlands. To detect IgG antibodies to Borrelia burgdorferi an enzyme-linked immunosorbent assay was performed. The prevalence of antibodies to B. burgdorferi in owners of hunting dogs was 15%, which was significantly higher than in blood donors (9%). In both groups the prevalence of seropositivity increased with age. There was no difference between men and women. The prevalence of antibodies to B. burgdorferi in blood donors from different regions varied significantly, ranging from 2% to 17%. Only 3% of the owners of hunting dogs could recall having had symptoms most likely to be due to Lyme borreliosis. Of the 68 seropositive hunters 64 (94%) were asymptomatic. The results of this study indicate that in the Netherlands asymptomatic infections with B. burgdorferi are common in individuals highly exposed to ticks and that there is considerable variation in the prevalence of seropositivity in the general population. Hence caution is called for in the clinical interpretation of IgG test results.