Although Borrelia burgdorferi is found at the site of many manifestations of Lyme disease, local infection may not explain all features of the disease. Previous work has demonstrated that the organism's flagellin cross-reacts with a component of human peripheral nerve axon, heat shock protein 60. The cross-reacting epitope is identified by a single anti-B. burgdorferi flagellin monoclonal antibody, H9724. We now report that the spontaneous and peptide growth factor-stimulated in vitro neuritogenesis of SK-N-SH neuroblastoma cells and other neural tumor cell lines is suppressed by H9724. In contrast, changes induced by exposure of these cells to optimal and suboptimal concentrations of cyclic AMP, phorbol ester, or retinoic acid are not affected by H9724. H9724 does not decrease cell viability or the ability of the cells to anchor to the culture plate or extracellular matrix and does not block nerve growth factor binding to the cells. These findings are compatible with the premise that antiaxonal antibodies formed during the immune response to B. burgdorferi flagellin might modify axonal function in vivo and play a role in the pathogenesis of neurologic features of Lyme disease. A humoral immune response predicated on molecular mimicry could explain persistent or ongoing neurologic dysfunction occurring after elimination of the organism by appropriate antibiotic therapy.