Borrelia burgdorferi, the causative agent of Lyme disease, appears during periods of growth to shed membranous material from its surface. Using an antigen capture/detection method developed in the laboratory, this material could be demonstrated on the surface of spirochetes, free in culture medium and in infected animals and ticks. Although the captured antigens were initially assayed by immune electron microscopy, other methods have been used recently to characterize the nature of these bioproducts and to assess their possible role in the pathogenesis of Lyme disease. Researchers in the laboratory have been able to demonstrate that extracellular components of B. burgdorferi 1) appear to be present wherever active growth of the organism is taking place and therefore, may be useful as a diagnostic indicator of active infection and/or treatment effectiveness; 2) are involved in the packaging and protection of intact DNA molecules containing a few known and many unknown genes and gene products; 3) appear to specifically interact with immunoglobulin M molecules in a unique fashion, perhaps to escape immune surveillance; and 4) possesses potent, non-specific mitogenic activity which may cause an inappropriate and noneffective stimulation of the immune system triggering autoimmune disease components. Researchers continue to examine these and other bioproducts with the aim of improving the prevention, treatment and diagnosis of Lyme disease.