To trigger class II-restricted T cells, antigen presenting cells have to capture antigens, process them and display their fragments in association with class II molecules. In most species, activated T cells express class II molecules; however, no evidence has been found that these cells can present soluble antigens. This failure may be due to the inefficient capture, processing or display of antigens in a stimulatory form by T-cells. The capture of a soluble antigen, which is achieved by nonspecific mechanisms in macrophages and dendritic cells, can be up to 10(3) times more efficient in the presence of surface receptors, such as surface immunoglobulin on B cells that specifically bind antigen with high affinity. We asked whether T cells would be able to present soluble antigens that bind to their own surface molecules. Here we show that such antigens can be effectively processed and presented by both CD4+- and CD8+-bearing human T cells. This indicates that T cells are fully capable of processing and displaying antigens and are mainly limited in antigen presentation by their inefficiency at antigen capture.