A group of 146 children with acute rheumatic fever and 256 members from their families, and a group of 125 children with post-streptococcal glomerulonephritis and 199 family members, together with a group of 145 children with non-complicated group A streptococcal pharyngitis were examined over a period of three years. The purpose of the study was to explore the concurrent association and distribution of group A streptococcal serotypes among the three groups. Strains isolated from the children with non-complicated group A streptococcal pharyngitis represented the prevalent strains of group streptococci in the childhood community during the period of study. Rheumatic fever was encountered in a non epidemic situation. As expected, the recovery of group A streptococci was low. The strains however belonged mainly to two patterns, namely type M1 and M non-typable strains. Nephritis was pharyngitis-associated and occured also sporadically throughout the year. Types M12 and 49 accounted for the majority of the isolates. Type M12 accounted for 34.4% of the group A isolates from family members of children with nephritis and was totally absent in family members of children with rheumatic fever (p less than 0.001). The T pattern 8/25/Imp. 19 accounted for 40% of the group A isolates from family members of rheumatic patients compared to 3.3% from family members of nephritis patients (p less than 0.005). Data from this study show that the group A streptococcal serotypes, concurrently isolated from children with acute rheumatic fever and their families are disparately different from those of children with pharyngitis-associated glomerulonephritis and their families in the same population. These findings support the concept of "nephritogenicity" and "rheumatogenicity" and indicated the important role of the biological characteristics of the streptococcal serotypes in the aetiology of acute rheumatic fever and acute poststreptococcal glomerulonephritis.