The author investigated the changes of serum protein, protein fraction and immunoglobulin in 6 infants who had been immunized with D.T. vaccine at 3, 4 and 5 months of age and in 6 infants who had not been immunized, as control group. The results obtained were summerized as follows. 1) There were no differences in the changes of total sserum protein and protein fraction between two groups. 2) Although, in the level of immunoglobulin, there were no differences in the levels of IgG and IgM between two groups, IgA was increased slightly in immunized group than in nonimmunized group.