An attempt was made to classify human pituitary cell types by electron microscopic immunohistochemistry. The immunoperoxidase technique involving the use of the peroxidase-antiperoxidase complex was applied to thin sections of human pituitaries removed surgically for breast cancer or diabetic retinopathy. Using specific antibodies against human PRL, GH, beta-FSH, beta-LH, beta-TSH, and porcine ACTH, the localization of each hormone was studied. Identification of 5 human pituitary cells was possible: 1) The PRL-secreting cell contains round or slightly ovoid secretory granules of a diameter of 275-350 nm. 2) The GH-secreting cell is densely granulated with granule diameters ranging from 350-500 nm. 3) The gonadotrophic cell, which stains for both beta-FSH and beta-LH, is characterized by the presence of a varying number of secretory granules ranging from 275-375 nm. 4) The cortico-lipotrophic cell has numerous granules of about 375-550 nm in diameter. 5) The TSH-secreting cell contains small secretory granules of about 125-200 nm in diameter. Another cell type of which the small secretory granules of about 100 nm in diameter could not be stained by any of the antisera was also observed. This ultrastructural identification of human pituitary cells should contribute to a better understanding of the pathophysiology of the human pituitary.