The total protein content in the blood serum of 416 species and subspecies of vertebrates is determined by the microbiuret method. Generally speaking the total protein content of the serum shows an increase with a higher evolutionary level. The difference between the mammals and the other groups is particularly large. This increase is probably attributable to a number of factors, for instance adaptation to living on land, development of the circulatory system, the development of adaptive immunity and the forming of specific transport proteins. A connection between albumin content and level of development can also be noticed.