In vitro modulation of human T lymphocyte surface differentiation antigens T3, T8, and T4, by their respective monoclonal antibodies, was studied as a function of donor age. Kinetic studies performed on lymphocytes from young adults indicated that modulation is dependent on concentration of antibody used and duration of culture. OKT3 modulates T3 rapidly (maximum at less than 24 hr) and relatively completely (79% at the highest concentration of antibody used). By 48 hr, regeneration of the T3 antigen is apparent. T8 modulation by OKT8 is slower (continued modulation at 48 hr) and less complete than is T3 modulation by OKT3. OKT4 does not modulate the T4 antigen. In elderly individuals modulation of T3 by OKT3 is preserved whereas modulation of T8 by OKT8 is significantly reduced (24 +/- 8% at 48 hr vs 53 +/- 4% for young controls). These observations document further age-related changes in properties of human T suppressor cells.