The cytokeratin pattern of medullary, mixed and cortical thymomas and thymic carcinomas were analyzed by two-dimensional equilibrium electrophoresis. Extracts from a medullary thymoma and normal, total thymi showed a similar pattern. On the other hand in cortical thymomas and very similar in thymic carcinomas there were marked changes in the cytokeratin pattern. These findings support the classification of thymomas as described by MULLER-HERMELINK.