The continued proliferation of activated T cells requires the presence of a lymphocyte growth factor in the culture medium. This study describes a rapid, highly reproducible assay to quantitatively measure levels of this lymphokine. The use of Concanavalin-A blast cells given this assay a high degree of flexibility and convenience. It is shown that the lymphokine measured is interleukin 2. The presence of an inhibitor in the supernatant of mitogen activated lymphocytes and the species specificity of the factor are demonstrated.