Abstract. Blastemal cells of embryonic mouse limb buds (day 12) were cultivated in organoid cultures in the presence of the human recombinant cytokines interleukin-1 (IL-1) and tumour necrosis factor (TNF-). The effects of both cytokines (applied alone or together) on mesenchymal cells were demonstrated by electron microscopy. Cultures treated with TNF- (alone or in combination with IL-1) showed several mesenchymal cells with numerous irregularly shaped membrane-bordered cavities containing thick bundled tannic-acid-positive fibrillar structures that resembled loosened collagen fibrils, whereas cells exposed to IL-1 alone did not exhibit such changes. These findings are discussed in the light of two hypotheses: the phagocytosis of extracellular collagen fibrils, and fibrillogenesis resulting from incongruity of synthesis and secretion rates of procollagen; our results favour the former.