A derivative of the hypothalamic peptide luteinizing hormone-releasing hormone (LHRH) has been coupled to ferritin and the conjugate purified by gel chromatography. In its ability to stimulate the secretion of luteinizing hormone from pituitary cells in vitro, the conjugate has the same potency and specificity as the native peptide. When dissociated pituitary cells maintained in short-term culture are lightly fixed with formaldehyde and then incubated with the conjugate, examination in the electron microscope shows an even distribution of ferritin particles over the free cell surface of the gonadotrophin cells. This binding appears to be specific for the LHRH receptor since it is prevented by a 10-fold excess of native peptide. In addition to the gonadotrophin cells, some somatotrophin and thyrotrophin cells bind conjugate on their free surfaces under similar conditions. If living cells are incubated with the conjugate for 15 min, the bound conjugate becomes aggregated and then concentrated in one localized area of the cell surface. In this area, which lies immediately above the juxtanuclear Golgi complex, the plasma membrane is frequently invaginated in a manner which suggests that the bound, aggregated conjugate is internalized by endocytosis.

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