The soluble cytoplasmic proteins isolated from bronchial mucosae and 15 bronchogenic carcinomas of human origin have been examined by agarose-gel electrophoresis and compared with a similar protein preparation isolated from the bronchial mucosa of cynomolgus monkeys (Macaca irus). In all cases, 3 bands were found, 2 (A and B) which migrated to the anode and one (C) which migrated to the cathode. The relative proportions of protein in the 3 bands were similar in smokers and nonsmokers, but there was a significant increase in the proportion of protein in band C in all of the bronchogenic carcinoma material examined. This increase was most pronounced in ‘oat’-cell carcinomas. The finding that basic proteins are increased in bronchogenic carcinoma is in contrast with the decrease in basic proteins in some experimental animal tumors; it is suggested that this difference might be related to functional differences between basic proteins in different tissues. The electrophoretic pattern of human and cynomolgus monkey material was similar, indicating that the cynomolgus monkey may be a suitable model for gaining experimental data of relevance to alterations in these proteins in human material.