Fibrous bodies in growth hormone-secreting adenomas were studied by immunocytochemistry using an anti-cytokeratin antiserum. Fibrous bodies are a feature of acidophil and chromophobe adenomas and are usually associated with acromegaly. Ultrastructural examination revealed round juxtanuclear filamentous aggregates, composed of 8-nm filaments and other organelles. This study demonstrated that the intermediate filaments of fibrous bodies belong to the cytokeratin class. The etiology and significance of these filamentous aggregates remain obscure.