The critical role of major histocompatibility complex (MHC) gene products in T cell activation was appreciated and extensively studied well before the availability of monoclonal T cell populations. However, the availability of cloned T cells has dramatically enhanced the ability to characterize the nature of MHC-restricted recognition by T cells. In certain areas, the use of monoclonal T cells has allowed substantial extension of principles already established through the use of heterogeneous T cells. In other cases, studies employing T cell clones have led to findings which were inaccessible to approaches using only heterogeneous T cell populations. On balance, it should also be pointed out that a significant number of critical questions concerning the MHC-restricted T cell repertoire remain best approached by studying the development of interactions of functionally heterogeneous T cell populations. The present review will focus upon recent progress in several areas in which the use of cloned T cells has been particularly important in characterizing the nature of MHC-restricted recognition. In particular, the restriction of T cell recognition by class II MHC products (Ia) is examined in detail.