Lymphocytes from 107 healthy individuals and 34 patients with lung cancer were cultured and stimulated with PHA. Wide variation in lymphocyte transformation reactivity was observed, ranging from significantly low responses to high blastogenesis. Low spontaneous blastogenic responses were not always associated with a simultaneous decrease in PHA-induced lymphocyte transformation in the same individuals. The results are presented by the (disintegrations per minute) values of the stimulated and nonstimulated cultures and by the stimulation index. A new index termed the Blastogenic Cumulative Index is proposed. It is defined as the sum of the logarithms of the values of the spontaneous and the PHA-induced lymphocyte transformation. A better relationship was observed in immunocompetent and in immunodepressed individuals between the dpm values and Cumulative Indices than between the dpm and the Stimulation Indices.