The rate of entry of radioactive rubidium (86Rb) into the erythrocyte in vitro was studied in 27 normal volunteers, 39 subjects with varying nonmalignant lung diseases and 38 subjects with biopsy-proven bronchogenic carcinoma. The rate of active uptake, as well as outward diffusion, was not statistically different between the normal and nonmalignant lung disease subjects. In contrast, these parameters were increased in the tumor-bearing subjects. In a limited number of subjects, these parameters were noted to return to normal values when a histamine and serotonin antagonist was added to the incubation mixture. In four cases, these parameters returned to normal following resection of the lung tumor. These studies are suggestive but not conclusive of an erythrocyte membrane defect which might be exploited in the development of a cancer screening test.