This Nobel Prize Lecture by Landsteiner, given at Stockholm, deals not only with the 4 blood groups for the discovery of which the prize was recently awarded, but also with individual differences of human blood in general. The 4 human blood groups are described and also the new factors, M and N, and the heredity and forensic application of both sorts of properties are discussed. These new factors, and also the sub-groups, increase considerably the variety of human blood and support the view that practically every human blood may be different from every other[long dash]a result previously established for some animal spp. Thus, a connection is established with the remarkable individual specificity of the trans-plantation of tissues. The substances underlying these individuality reactions are, in all likelihood, not proteins. The latter part reviews the subject of the use of blood groups for blood transfusion.