AbstractAdjacent semithin and ultrathin sections were used to correlate specifically immunolabeled cells at the light microscope level with the superimposed subcellular compartments at the electron microscope level. Prolactin immunoreactivity was successfully located in theparaformaldehyde-lysine-periodate (PLP)-osmium fixed, plastic embedded rat anterior pituitary glands at both the light and electron microscope levels. Immunofluorescence revealed slightly heavier immunoreactive product in the unosmicated tissues than in the osmicated. By superimposing semithin section and consecutive ultrathin section, the brightly immunofluorescent areas appeared to be secretory granules. This method may be valuable for identifying the fine structure of specific cells immunolabeled by various hormonal antigens, protein markers, and putative neurotransmitters other than in the pituitary gland. (The J Histotechnol16:125, 1993)