Tubuloreticular structures (TRS) were observed electron microscopically in stroma cells of human laryngeal carcinoma. The TRS appeared as networks of membranous tubules 20-30 nm in diameter, which were always situated in dilations of the endoplasmic reticulum (ER). The presence of TRS was observed not only in cells situated in the tumour stroma, but also in some stroma cells which had migrated to the carcinomatous epithelium. However, they were not found in malignant keratinocytes. In some places, incomplete forms of TRS may be observed, which in earlier reports have been referred to as worm-like or comma-shaped bodies. These incomplete forms of TRS, which are clearly shown to arise contiguous to the ER cisternae, may represent different developmental stages of TRS morphogenesis rather than disintegration steps. The observations indicate that the TRS - seen in various disorders such as viral, neoplastic, and auto-immune diseases - may well represent a common reaction pattern of the ER system of the host cell.