The frequency and specificity of antibodies that bind antigens of Borrelia burgdorferi in sera from 200 individuals with no evidence of past or current Lyme disease was determined. Sera were tested for both IgG and IgM antibodies to B. burgdorferi by Western blotting. The non-Lyme serum group included specimens from healthy adults and children in addition to specimens from patients with viral infection and rheumatic diseases. Crossreactive IgG antibodies occurred more frequently than IgM antibodies. The most frequently bound antigens corresponded to 41 kDa and 60 kDa Borrelial components. Of 200 specimens tested, 100 had antibodies that bound at least 1 antigen. Binding to multiple antigens occurred at much lower frequency. Our results indicate that determination of maximum crossreactivity of non-Lyme sera can be used to establish minimum criteria for determining a positive Western blot result for Lyme disease.