Lymphocytes of 40 cancer patients with solid tumors as well as of 40 normal subjects were stimulated by phytohemagglutinin (PHA), purified protein derivative (PPD), streptokinase-streptodornase (SK-SD) and candidin (Cand) and the extent of their stimulation was expressed by the index of reactivity. The cumulative frequency distribution of reactivity indices was significantly higher in normal individuals than in cancer patients. The highest extent of information relevant to the discrimination between patients and normal subjects is obtained following stimulation of lymphocytes with Cand and in decreasing order with PPD, SK-SD and PHA. The percentage of normal subjects and patients having reactive lymphocytes was found to be the highest following stimulation with PHA, lower with PPD and the lowest to Cand and SK-SD. The importance of using increasing numbers of stimulants to achieve maximal detection of lymphocyte reactivity is documented and discussed.