Fluorescein (FIT) labelled IgG fractions from patients with acute poststreptococcal glomerulonephritis (AGN) stain parts of the basement membrane and mesangium of glomeruli from the same patients, provided renal tissue is obtained early in the disease. Staining is abolished by preabsorbing the IgG fractions with disrupted streptococci isolated from patients with AGN. Non-nephritogenic streptococci do not reduce staining. These findings were applied in an epidemiologic survey of a pediatric population with group A streptococcal infections. During a 12-month period 178 children with group A streptococcal infections were followed with weekly examinations including urinalysis and determination of serum complement (CH50) and ASLO titers. Only children in whom these parameters were normal initially were kept in the study. 2 children developed typical AGN. 21 patients were asymptomatic but showed transient urinary abnormalities and decreased CH50. Their renal tissue showed glomerular lesions ranging from mild mesangial increase and endothelial cell proliferation to the characteristic changes seen in AGN. The glomeruli showed granular staining with FIT labelled anti-human IgG and β1C. Streptococci cultured from the children were preserved. Only bacteria isolated from patients with demonstrable glomerular lesions reduced the staining capacity of FIT labelled sera from patients with AGN. Streptococci obtained from patients without evidence of renal involvement failed to preabsorb. It appears that nephritogenicity of streptococci can be predicted. This study suggests that incidence of glomerular damage following group A streptococcal infections is greater than suspected.