Fat embolism was induced in rabbits by giving an intravenous injection of radioactively labelled homologous retroperitoneal fat. One group was defibrinogenated by Arvin. A second group was made thrombocytopenic by treatment with Busulphan. After the animals had been made thrombocytopenic and had been defibrinogenated, respectively, fat embolism was induced. Macroscopic and histologic examinations were carried out, as well as recordings of wet weight and radioactivity content of the lung. There was a highly significant increase in the pulmonary wet weight of thrombocytopenic animals compared with controls. Macroscopic as well as histologic examination revealed a massive interstitial and alveolar oedema. Half of the specimens showed moderate to massive bleedings. Defibrinogenated animals did not differ from controls. Under the given circumstances, the results suggest that platelets are protective to the endothelial lining of the pulmonary capillaries during embolism. The presence or absence of fibrinogen initially does not seem to be of major importance for the pulmonary damage induced by fat injection.