Clusters of chief cells from a fragment of human parathyroid adenoma possess relatively few rough endoplasmic reticulum (RER) cisternae with scattered annulate lamella (AL) pores in cytoplasmic sectors close to the nucleus. Some of these cisternae exhibit winding profiles with smooth segments lying very close to the outer nuclear membrane. Other groups of cells exhibit well-developed stacks of RER either in continuity with or separated from AL. Two other tumor fragments show chief cells with few RER cisternae scattered in the cytoplasm and possess stacks with various amounts of AL and/or paired cisternae (PC), occasionally at perinuclear sites. Compartmental continuity between AL and PC is a frequent finding. Pore density in AL varies considerably between the stacks from different cells.