The 80K (80 x 10(3) Mr) integral membrane protein, first described in the secretory granules of rat basophilic leukaemia (RBL) cells, is also localized to lysosomes in these cells. The protein displays the same distribution in natural killer lymphocytes (RNK-7), wherein it codistributes with cytolysin in secretory granules. In contrast, the protein is absent from the endocrine and exocrine secretory granules of rat pancreatic acinar and pituitary cells, respectively, where it is confined to lysosomes. The protein colocalizes with lysosomal integral membrane proteins in all the cells studied, indicating that is largely restricted to secretory granules with lysosomal properties (LSG) and lysosomes. The protein expressed on the surface of secreting RBL cells is internalized by endocytosis via coated pits, and found in coated vesicles, endosomes, multivesicular bodies and Golgi system, before being recycled to LSG and partly delivered to lysosomes. The recycled protein is re-expressed on the surface of cells stimulated to secrete a second time.