The mitogens phytohemagglutinin (PHA) and concanavalin A (Con A) stimulate only those mouse lymphocytes which have undergone differentiation within the thymus and thus bear the differentiation antigen θ. However, not all thymus-derived lymphocytes (T cells) react equally to both mitogens, and the differential responsiveness of T cells to PHA and Con A may be used to indicate the existence of T cell subsets. One subpopulation of T cells exists which demonstrates approximately equal reactivity to both mitogens and bears a relatively high density of θ determinants. Another subpopulation exists which responds mainly to Con A, and bears a relatively reduced density of θ determinants. T cell subsets so delineated also differ in their recirculation patterns, radiation sensitivities, location in peripheral lymphoid tissue, and most importantly, in their function. These differential characteristics may represent the capabilities of T cells in various stages of differentiation within one T cell line. On the other hand, these cells may be representative of two distinct T cell lines and preliminary evidence consistent with this possibility is presented.