The authors, using passive hemagglutination, looked for B. burgdorferi antibodies in animals against 2 strains of this Borrelia, each belonging to one of the 2 most important groups of isolates currently known. Antigens were prepared respectively from the strain FR 20047 isolated in Brittany, and from the strain B31, a reference american one, also existing in Europa. Sera from 723 mammals (wild : foxes, deers, wild-boars, field-mice, bank-voles, etc... ; domestic : horses and dogs) were tested, giving 189 seropositives with significative titles (greater-than-or-equal-to 1/100) with one or (and) other from the 2 antigens parallely used. Comparison of the titles obtained against the 2 antigens showed in some cases important differences of serological reactivity, and this inside a same mammal species, according to, for example, the geographic origin. The fact that only one strain as antigen does not permit to reveal significative seropositivities in all infected animals leads us to think that using antigens made from strains belonging to different groups of isolates is useful for a valuable serological diagnosis of this borreliosis in animal species as well as, probably, in man.