The elevation of complement level in the sera and depressed state of tuberculin reaction were observed in lung cancer patients. A clinical follow-up study demonstrated negative conversion of tuberculin reaction while keeping the complement at an elevated level during the observation period. This phenomenon can be explained; the complement system is elevated to compensate the depressed cell-mediated system to prevent the immunological surveillance system from invading agents in tumor bearing hosts. The immunological states of the patients with various diseases are classified into six stages according to the tuberculin reactivity, positive or negative, and complement level: elevated, normal, or depressed. A healthy control group is composed of the group of complement normal and tuberculin positive (Stage I). Most of acute inflammation falls into the elevated level of both complement and positive tuberculin reaction (Stage II). Sarcoidosis, leprosy, and Wegener's granulomatosis are divided into the elevated level of complement and depressed tuberculin reaction (Stage III). Systemic lupus erythematosus is in Stage V with the depressed state of both tuberculin reaction and complement level. A follow-up study of lung cancer patients showed a possible chronological sequence starting from Stage I through III, and finally to V, similar to the progression-of-disease process. The biological and medical significance related to the phenomenon is discussed, standing upon immunochemical, phylogenical, and immunogenetical standpoints of complement research.