Tumour cell plasma in 4 out of 8 bronchial carcinoids was found to contain sphere-shaped formations of microfilaments different in nature from tonofilaments, myofilaments or amyloid fibrils. The filamentous structures were closely related to agranular endoplasmic reticulum with the occassional inclusion of endocrine granules of type Pa cells. In the discussion prt, the author refers to similar structures in other tumours arising from cells of the diffuse endocrine system, and in some of the lung carcinomas.