Enzyme-linked immunosorbent assays (ELISAs) with separate preparations of 10 purified recombinant antigens of Borrelia burgdorferi sensu stricto were used to test seta from 36 dogs not vaccinated with whole cells of this agent and from five dogs vaccinated with whole-cell B. burgdorferi bacteria. All dogs lived in tick-infested areas of Connecticut and south-eastern New York state, USA. The non-vaccinated dogs had limb or joint disorder, lameness and fever during the period 1984-1991 and had antibodies to B. burgdorferi, as determined by a polyvalent ELISA with whole-cell antigen. In reanalyses of seta for total immunoglobulins in ELISAs with recombinant antigens, reactions were most frequently recorded when outer-surface protein (Osp) F, protein (p)35, p37, p39 and p-41G (a flagellin component) were tested separately. Western immunoblots of a subset of 16 sera, positive by ELISA with whole-cell antigen and representing a range of antibody titres (640-40 960), verified immune responses to these or other lysed whole-cell antigens. Sera from vaccinated dogs contained antibodies to OspA, OspB, p22, p37 and p41-G. Therefore, serological reactions to OspF, p35 and p39 were the most important indicators of natural exposure to B. burgdorferi. Serum reactivities to these recombinant antigens in ELISAs can be used to help identify possible natural infections of canine borreliosis in dogs not vaccinated with whole-cell B. burgdorferi and to provide information on the geographic distribution of this bacterium.