The question of detecting the changes in reactivity after streptococcal infections and before the first rheumatic attack is of great importance from the standpoint of rheumatology. This paper reports the results of our serological investigations after scarlet fever in a two-year follow-up of 31 children. Examinations, using nine laboratory tests, useful in rheumatology as indices of inflammation and immunobiological equilibrium, were carried out.From the beginning of 1957 to the middle of 1959, 31 children with scarlet fever were treated for six days with penicillin and were followed up clinically and with laboratory tests. Sixteen of them were examined in the first 10 days of the disease, and tests were carried out later, after complications, in 15 cases.Five children were 4 or 5 years old at the time they caught the disease, 23 were between 6 and 10 years, and three were between 11 and 13 years of age.