Human peripheral blood T-lymphocytes were fractionated by discontinuous density gradient centrifugation on Percoll. T-cells of low density were capable of proliferation in the presence of the mitogen phytohemagglutinin (PHA) as measured by colony formation, whereas cells of high density were not. Addition of supernatant of PHA-stimulated cultures to cells of high density resulted in proliferation which suggested that these cell fractions lacked interleukin-2 (II-2) producing T-helper cells. This finding was supported by the demonstration that T-cells of high density did not produce II-2 but could be induced to do so by adding II-1 producing monocytes. The data area explained on the basis of the present model of intercellular events leading to activation and proliferation of lymphocytes.