Children with energy-protein undernutrition showed a reduction in the number of circulating T lymphocytes identified on the basis of their ability to form rosettes with sheep red blood cells. T cells with a receptor for IgM (Tmu) were decreased whereas T cells with a receptor for IgG (T gamma) were increased. Surface immunoglobulin bearing B cells were comparable in well nourished and malnourished subjects but the proportion of B alpha was increased in the latter. "Null" cells without the conventional markers of T or B cells were proportionately increased. Leukocyte terminal deoxynucleotidyl-transferase activity was elevated in the majority of undernourished children and correlated with the proportion of "null" cells. The significance of these observations is discussed and it is suggested that "null" cells represent immature undifferentiated T lymphocytes.