CST-3 is a secretory granule protein of anterior pituitary cells. Frozen thin sections of rat pituitary gland were stained for immunofluorescence using rabbit antibodies to CST-3 and Alexa 488-conjugated anti-rabbit IgG (green). Guinea pig antibodies to (A) insulin (as a nonspecific control), (B) Prl, (C) FSH, (D) thyroid-stimulating hormone (TSH), and (E,F) GH as well as Alexa 594-conjugated anti-guinea pig IgG (red) were used to identify the various pituitary cell types and label the granules within them (arrowheads). Antibodies to CST-3 labeled the majority of the cells (arrows), which overlapped (yellow) with those also labeled by antibodies to FSH, TSH, and GH but not Prl. High magnification matched images from sections incubated with anti-CST-3 (G,J) together with anti-FSH (H) or anti-TSH (K) show good correspondence between the staining for CST and the granule proteins (I,L), indicating that CST-3 is in secretory granules. Bar = 5 μm.