In addition ot Borrelia burgdorferi, recognized as the aetiological agent of Lyme disease, at least two separate genospecies have recently been described. A relationship between infection by strains belonging to different genospecies and clinical outcome has been suspected. In this paper, 9 cases of Lyme arthritis were attributed to infection by B. burgdorferi sensu stricto, 18 cases of neuroborreliosis to B. garinii and one case of acrodermatitis chronica atrophicans to a strain of B. afzelii. These conclusions were based on the preferential reactivity of sera with antigens of given strains in Western blots and on residual reactivity after absorption of sera with antigens of representative strains. No conclusion could be reached concerning sera of 10 patients with erythema migrans.