There are few satisfactorystaining methods for fibrin. There are even fewer clear definitions of the material for which these methods are commended. In sections of tissue the older pathologists recognized three phenomena to which they gave the name fibrin. These were:(a) The networks and more solid masses within blood vessels that formed a large part of the substance of what had been seen with the naked eye as a thrombus;(b) the extra-vascular networks of somewhat comparable structure associated with acute inflammation and seen in the interstitial tissue, in the arachnoid space, or in pneumonic alveoli; and (c) the more solid and plaque-like masses occurring onserous surfaces. When observations came to be made on the hyaline eosinophilic fibrin-like material found to be present in pathological processes other than simple coagulation and the pyogenic lesions, the names given to it were curiously varied. It is a measure of this vagueness that in tissues from rheumatic disease of the heart the eosinophilic deposit on the epicardial surface was called'fibrin', a comparable deposit in the superficial layers of the mitral valve was called'platelets', andthe eosinophilic and slightly refractile network of the reticular Aschoff body was called'fibrinoid'. Within recent years, almost contem-poraneously with the falling incidence of acute pyogenic inflammation, there has been an increasing recognition and experience of this fibrin-like material which, with the classical methods for staining fibrin, gives varied degrees of positiveness; typical examples are the intra-alveolar networks in the lungs of subjects with uraemia or cardiac failure, and the deposits of hyaline materials within the walls of blood vessels ('fibrinous vasculosis', Lendrum, 1955, 1956) as seen in systemic and pulmonary hypertension, in areas of infarction, and in the diseases which for want of more exact knowledge we group under the term'sensitization'. It is fashionable still to call this material'fibrinoid'but the new methods for demonstrating fibrin described below all give a sharp, contrasty snining reaction with this material, a finding which encourages us to abjure this melodious but misleading term. The fact of identical staining reaction on these