The terms fibrinoid degeneration and fibrinoid substance were first introduced by Neumann in 1880 to describe some alterations in the staining characteristics of collagen. Collagen fibres which have undergone this change are eosinophilic, homogeneous and take some of the tinctorial properties of fibrin. Lendrum et al. (1962) developed Martius-Scarlet-Blue (MSB) stain, which is preferentially taken up by fibrin. The principle of the method described by Lendrum and his colleagues is the use of acid dyes of different molecular size (Martius yellow, Brilliant Crystal Scarlet 6R, methyl blue) in accordance with the alteration in structure of fibrin at different stages of development. The newest fibrin likely to be found in sections is presumably the fine network of postmortem fibrin. Much of this takes a yellow stain with the MSB method, as do erythrocytes. Slightly older fibrin (16 hours) takes a bright red stain with the MSB. Complete validity of the method depends on prolonged fixation in formal-sublimate. It gives brilliant staining with acid dyes and enhances metachromasia. In the Department of Forensic Medicine in Leeds it was noticed that an alteration in collagen staining with MSB stain was taking place in samples collected from different kinds of injuries, e.g. ligature marks, electrical injuries and burns. Similar changes were only occasionally seen in abrasions. It was therefore decided to inflict different kinds of injuries on the skins of rats and to fresh pieces of human skin in vitro, to observe any alterations in collagen staining with MSB and to assess the significance of these changes in relation to the timing of injuries.(ABSTRACT TRUNCATED AT 250 WORDS)