Palatine tonsil is well recognized issue as source of infection and origin of secondary disease such as nephritis and rheumatic fever. The author studied the correlations between the routine laboratory test including ASO titer and the changes of tonsillar histopathology with the number of pyroninophilic cells on the cases indicated for tonsillectomy. The following results are obtained ; 1. ASO titers measured before tonsillectomy are fairly well pararelled with the number of pyroninophilic cells in tonsils especially with that of mature forms. 2. ASO titers show higher values in those cases having more epithelial hyperplasia, lymphocytic and plasma cell infiltration, irregulatity of crypts and subepithelial young connective tissue. While those cases with more neutrophile infiltrations, markedly increased capsular connetive tissue and follicular atrophy show relatively low ASO titer. 3. In cases suggesting nephritis in routine laboratory examination with albuminuria shows markedly increased ASO titers and pyroninophilic cells. 4. Mean ASO titers show highest value in the age group of 11 to 15 yrs. and the tonsillar hyperplasia and the increase of pyroninophilic cells are also highest in this age group. 5. There are some cases showing markedly increased nember of pyroninophilic cells with low ASO titer and these cases are suggestive of infections other than streptococcus. The value of ASO titer and the number of pyroninophilic cells in tonsil seems to be related with time lapse after streptococcal infection, antibiotics treatment and the degree of recurrent infections of tonsil.