IgD is a surface immunoglobulin, which is coexpressed with IgM on greater than 90% of mature B cells, but its levels in serum are extremely low compared to those of IgM. It role as a surface receptor has been reemphasized by our recent findings that IgD receptors are induced on helper T cells by exposure to IgD and that such cells have immunoaugmenting properties. The present study shows that crosslinking of soluble IgD or of monomeric cell surface IgD is required and sufficient for the induction of T cells bearing receptors for IgD, both in vivo and in vitro. Effective IgD crosslinking in this respect can be obtained with antigen or with heterologous and immunogenic as well as nonimmunogenic allotype-specific anti-IgD. These results reinforce the concept that the induction of T cells bearing receptors for IgD is an integral component of the normal immune response.