An active subpopulation of peripheral blood T lymphocytes, characterized by rapid (5 min) rosette formation with sheep erythrocytes (A RFC) was measured in normal individuals after they were skin tested with microbial antigens. A significant rise in A RFC occurred in all individuals who developed positive delayed cutaneous hypersensitivity (DCH) reactions, whereas skin test nonresponders showed no significant rise in A RFC. No similar consistent changes occured in populations of total T cells, characterized by longer (60 min) rosette formation with sheep erythrocytes, or in B cells, measured by immunofluorescence of surface immunoglobulin. The A RFC response paralleled the DCH response in timing, but not in intensity. The results provide in vivo evidence for a biologically distinct T cell subpopulation, and focus attention on the A RFC as immunologically active cells.