Lymphocyte cell-surface markers were examined in forty children with acute rheumatic fever (ARF) and twelve with acute post-streptococal glomerulonephritis (AGN) and compared to thirty-six normal controls of similar age. Cell-surface-marker studies included surface Ig using fluorescein-labelled F(ab)2 anti-F(AB')2, IgG aggregate binding cells, and EAC rosettes. T cells were identified both as 'active' rosettes and total E-binding cells. Proportions and absolute numbers of cells bearing surface Ig and Fc receptors were elevated in subjects with AGN (Pless than0-01-0-5), whereas proportions of cells producing EAC rosettes were diminished. Patients with acute rheumatic carditis or chorea showed a substantial elevation in proportions and numbers of active T-cell rosettes (Pless than0-01). Streptococcal antigen binding cells capable of forming rosettes with autologous cells coated with group A streptococcal membranes were elevated in the acute phase of both rheumatic fever and acute glomerulonephritis(Pless than0-01). The majority of such cells were removed by passage over insolubilized Ig-anti-IgG columns and appeared to be B cells.