The problem of the nature of the blood covenant (also called blood pact or blood brotherhood) still awaits adequate detailed comparative analysis. This is unfortunate since detailed consideration of such practices would be interesting in itself and might also provide valuable means for understanding some of the notions of physiology, kinship, magic, and alliance in the societies in which this institution exists. A blood covenant establishes a close bond between two persons not linked by kinship but who, nevertheless, desire social relations supported by stronger sanctions (physiological and/or magical) than those afforded by mere communal, tribal, or trading connexions. It is clear that blood covenants do not establish conventional kinship relations and that the term ‘blood brotherhood’ is misleading. However, the physiological basis of these covenants and the terminology sometimes employed by the persons involved in them do suggest that some comparison with kinship may be useful if only to indicate the differences between these two institutions. By examining some of the ways in which a people's blood-covenant relations are similar and dissimilar to certain of their kinship relations, and by considering the significance which the people themselves see in the physiological link of common blood, we may gain a clearer understanding of both these types of relations.