Several monoclonal antibodies (mAbs) demonstrate further heterogeneity of CD4 and CD8. Within CD4, two major subsets can be identified with helper/inducer and suppressor/inducer functions. It is argued that suppressor/inducers, identified by CD45R mAbs, are immature or virgin T cells and therefore respond poorly to recall antigens. Helper/inducers are CD45R negative but express the UCHL1 antigen, which appears to be part of CD45. These cells respond well to recall antigens and are therefore mature or memory T cells. Several antigens not related to CD45, such as 4B4, Leu8, TQ1 and 9.3, identify functionally distinct subsets rather than stages of T cell maturation.