Ceruloplasmin was assayed as enzyme activity, as antigen, and as total copper in serum samples from 150 male lung cancer patients and comparable numbers of male controls. By all three assays, ceruloplasmin was significantly increased above the normal before treatment, and the degree of elevation was related to TNM stage [i.e., the International Union Against Cancer classification system based on extent of primary tumor (T), condition of lymph nodes (N), and absence of presence of metastases (M)]. Surgery had no immediate effects, but in patients who evidence of disease for longer periods, ceruloplasmin returned to nearly normal values. High levels of ceruloplasmin was elevated in 6 of 9 patients before tumor recurrence; 2 of 3 smokers (in the first panel of sera) with elevated ceruloplasmin levels subsequently developed lung cancer. The relative merits of the three assays were compared. Some sex- and age-related differences among normal controls were apparent. The results of pilot studies on men with gastrointestinal cancer and women with breast cancer are presented. It is concluded that only in limited situations will assays of ceruloplasmin aid in diagnosis, prognosis, and long-term monitoring of cancer patients.