Growth factors induce cell proliferation and are implicated in the multistep process of tumorigenesis. Transforming growth factor-alpha (TGF alpha), a peptide that binds to the epidermal growth factor receptor, is expressed by carcinomas and normal tissues. To investigate the possible role of TGF alpha in adenohypophysial tumorigenesis, we studied its expression in nontumorous human pituitary and different clinically and morphologically characterized human pituitary adenomas. Ribonucleic acid was reverse transcribed and amplified by polymerase chain reaction; transcript signals were identified with marked variation in 14 of 15 adenomas, and a weak signal was detected in nontumorous pituitary. Immunohistochemical positivity was found with variable intensity in all adenoma types, but not all tumors. Ultrastructural immunogold localized TGF alpha in endoplasmic reticulum, in Golgi apparatus, and on cell membranes; surface localization was confirmed by immunofluorescence. To assess possible secretion, the reverse hemolytic plaque assay was performed; small plaques were identified using an antibody that recognizes the extracellular domain of pro-TGF alpha; however, the plaques did not increase in size with time, suggesting that they detected membrane-anchored TGF alpha. Moreover, TGF alpha was undetectable by enzyme-linked immunosorbent assay in pituitary tumor-conditioned culture media. The marked variable expression of TGF alpha, the absence of secretion in measurable quantities, and the preferential membrane localization suggest a specific juxtacrine mechanism for TGF alpha in pituitary tumorigenesis.