In a group of 48 children the following streptococcal antigens were tested by means of the lymphocyte transformation test: group A streptococcal M1- and M19-protein, mucopeptide, polysaccharides from group A and group C streptococci, erythrogenic toxins and streptokinase. Specific transformations were obtained only by M-proteins. Erythrogenic toxines induced an unspecific stimulation comparable to the mitogenic activity of phytohaemagglutinins. No reactions could be found with streptococcal mucopeptide, the group-specific carbohydrates of groups A and C-streptococci and streptokinase, resp. A significant difference was seen in the group of young children (mean age 5 years) between the specific transformability by M1-protein (a common type) and M19-protein (a rare type). No difference was seen with both M-proteins in the group of older children (mean age 11 years). In both groups of children no differences exist in the number of positive cutaneous reactions after injection of M1- and M19-proteins, but the intensity of reactions (diameters of erythema) were more pronounced with the M-protein of the common type 1.