We demonstrate here the presence of a distinct mature CD4+8- T cell subset in mouse thymus. This subset, termed "Thy0," is delineated by the absence of 3G11 expression from about half of the 6C10-/HSAlow/- fraction of CD4+8- thymic cells. Thy0 is detectable from the neonatal period and largely contributes the Th0-type diverse cytokine production previously reported for the HSAlow/-CD4+ thymic population. Further, cells expressing the T cell receptor V beta 8 gene family are found at increasing frequency in Thy0 with age, comprising 40-60% of Thy0 in adult BALB/c mice. This alteration of V beta 8+ cell frequency is unique to Thy0, since no other CD4+ subset in thymus or spleen shows such V beta 8 overusage. All functional CD4+ T cell subsets, including Thy0, show appropriate V beta clonal deletion associated with endogenous superantigens. Thus, it appears that Thy0 is an intrathymically generated secondary cell subset produced after CD4+ T cell selection.