The antibody response to Borrelia (B.) burgdorferi, and to measles virus as control antigen, was analysed by agarose isoelectric focusing (AIF) and immunoblot of joint fluid and serum from 10 patients with Lyme arthritis and 10 controls with rheumatoid arthritis. Among the Lyme arthritis patients, six had oligoclonal anti-B. burgdorferi IgG antibody bands in joint fluid and corresponding serum, one patient had oligoclonal antibody bands in joint fluid only and also an elevated B. burgdorferi-specific joint fluid to serum antibody ratio as evidence of intra-joint production of specific antibodies, and the remaining three patients were negative for oligoclonal-specific antibody bands. Absorption with B. burgdorferi antigen confirmed the specificity of the oligoclonal antibody bands. They comigrated only partially on AIF with oligoclonal bands of total IgG, and the specificity of most oligoclonal IgG in joint fluid and serum in Lyme arthritis remains undefined. Among the controls, no anti-B, burgdorferi IgG antibodies were detected by AIF and immunoblot. Instead, 9 of the 10 rheumatoid arthritis patients had oligoclonal anti-measles IgG antibody bands which were restricted to the joint fluid in three of them, indicating local production. We conclude that Lyme arthritis is often accompanied by an oligoclonal specific antibody response in joint fluid and serum simultaneously, and occasionally by intrasynovial synthesis of oligoclonal-specific antibodies.