Specific unresponsiveness can be induced in neonatal and adult BALB/c mice by antibody against antigen-specific receptor (antireceptor antibody). When heterologous antireceptor antibody is used in the indirect fluorescence technique, the number of fluorescent cells in these animals is significantly lower than in normal animals. Fluorescent cells appear after a relatively brief incubation of cells from adult-suppressed animals, whereas no fluorescent cells are detected when cells from neonatally treated animals are incubated briefly. Evidently, treating neonatal mice with antireceptor antibody specifically depletes the antigen-responsive clone. In contrast, antireceptor antibody causes reversible blockade of responsive cells in adult-suppressed animals.