Prompt, accurate diagnosis of gamma globulin deficiency as a cause of unusual susceptibility to infection is extremely important for several reasons: first, replacement therapy with pooled normal gamma globulin is effective in the prevention of serious infections if initiated before structural damage has occurred; second, supplies of gamma globulin should not be wasted on patients whose susceptibility to infections is due to other causes; and third, patients should not be subjected to the discomfort and possible hazards of repeated large intramuscular injections of gamma globulin except upon clear-cut indications. Significant hypogammaglobulinemia occurs in 3 main forms: (1) in atransientform, as a prolongation of the normally occurring depression of gamma globulin levels between the second and fourth months of infancy; (2) in acongenitalform, usually in boys, with sex-linked recessive inheritance; and (3) in anacquiredform in both sexes at any age, either as a "primary" disease