Reported frequencies of peripheral blood autoantigen-specific cells in autoimmune diseases are typically low, which could be due to true scarcity or to limitations of in vitro assays. In the present study, antigens were targeted to the antigen-presenting cell (APC) to enhance T cell proliferation, using an antigen delivery system (ADS), consisting of biotinylated anti-IgG, streptavidin and biotinylated antigen. This was able to bind B cells and monocytes and was internalized within 24 hours. T cell proliferation to tetanus toxoid was at least doubled using the ADS compared to conventional assay with antigen in simple solution. To evaluate the ADS in an autoimmune disease, we determined T cell responses to the insulin-dependent diabetes mellitus (IDDM)-associated autoantigen IA-2ic in patients with recent-onset IDDM. When IA-2ic was available conventionally in solution, proliferation was poor, but significantly higher in IDDM patients than control subjects. However, the ADS significantly enhanced proliferation by a mean 3-fold for all subjects, while maintaining the significant difference between IDDM patients and healthy controls. Increases in T cell proliferation via the ADS were due to the recruitment of approximately 3 times the number of CD4 + T cells stimulated in conventional assays. B cell depletion abolished enhancement suggesting that the ADS operates through recruitment of B cells as APCs. This flexible modification of the T cell assay offers greatly enhanced sensitivity for determining the frequency of antigen and autoantigen-reactive T cells.