Group-specific substances in sweat: a consideration of the medico-legal value of absorption methods applied to sweat-stains on clothing, with an account of the results obtained in fifty experimental cases. The development of medico-legal blood grouping methods and their limitations are reviewed briefly. It is pointed out that the value of blood- grouping evidence is very frequently minimised or completely nullified by the absence of any evidence as to the blood group of an accused person. The record of an actual case is presented in which sweat staining on the clothing of an accused person was of importance, in that it not only interfered with the grouping of the victim' blood stains, but also proved of positive value in establishing the blood group of the accused. The results of a number of preliminary experiments are described and discussed. These experiments, together with the case referred to above, favour the impression that, in a significant proportion of cases, it might prove practicable to determine the blood group of an individual from an examination of his sweat stains. Previous work and recorded opinions on the presence of group-specific substances in sweat are reviewed briefly. From this review, it appears that there has been comparatively little work done, and that little or no use has been made of sweat stains for medico-legal purposes, certainly in this country. The methods adopted and the results obtained in a series of fifty experimental cases.are recorded in detail, and the results are presented also in a summarised form. The results of these experiments confirm the belief that in a substantial proportion of cases it is possible to establish the blood group of an individual, within certain limits, by the investigation of the sweat stains on his outer clothing by relatively simple absorption methods.