Lymphocytes in the peripheral blood of young children with protein-calorie undernutrition were evaluated for surface markers and function. Thymus-dependent lymphocytes were reduced and the immunoglobulin-bearing B lymphocytes were unchanged. The relative proportion of the remaining "null" lymphoid cells was increased. Null cells and, to a lesser extent, B lymphocytes showed cytotoxic activity against xenogeneic target cells and suppressed phytohemagglutinin-induced DNA synthesis by normal T lymphocytes. It is suggested that these alterations in lymphoid subpopulations contribute to depressed cell-mediated immunity in malnutrition.