To find out whether apparent seronegativity in patients strongly suspected of having Lyme disease can be due to sequestration of antibodies in immune complexes, such complexes were isolated and tested for antibody to Borrelia burgdorferi. In a blinded analysis the antibody was detected in all 10 seronegative Lyme disease patients with erythema chronicum migrans (ECM), in none of 19 patients with other diseases, and in 4 of 12 seronegative patients who probably had Lyme disease but had no ECM. These findings were confirmed by western blot, which also showed that immune complex dissociation liberated mainly antibody reactive to the 41 kD antigen and sometimes antibody to an approximate 30 kD antigen. Complexed B burgdorferi antibody was also found in 21 of 22 (95%) of seropositive patients with active disease, 3 additional seronegative but cell mediated immune reactive patients, and 3 other seronegative patients who eventually became seropositive. Apparent B burgdorferi seronegativity in serum immune complexes may thus be due to sequestration of antibody in immune complexes.