The effect of age on the ability of B lymphocytes and thymus cells from donors of various ages to be capable of producing an anti-idiotype-blocked, hapten-augmentable PFC was studied by adoptive cell transfer techniques. Lethally irradiated mice were reconstituted with syngeneic B lymphocytes and thymus cells from donors of various ages. Recipients were immunized with trinitrophenylated bovine gamma globulin (TNP-BGG) one or seven days after cell transfer. Splenic IgG anti-TNP plaque-forming cell (PFC) responses were assayed in the absence and presence of hapten for anti-idiotype (Id)-blocked, hapten-augmentable PFC, 14 days after immunization. It was found that the B lymphocyte population from 2 month old donors together with thymus cells from donors of various ages (2 to 19 months) were incapable of reconstituting mice to produce anti-Id-blocked, hapten-augmentable PFC. Similar results were obtained when mice were reconstituted with thymus cells from 2-month-old donors together with B cells from donors of various ages (2 to 14 months). In contrast, mice reconstituted with B cells plus thymus cells from the same 8-month or older donors produced a significantly high percentage of anti-Id-block, hapten augmentable PFC. Mice reconstituted with B cells from 8 months or older donors plus thymus cells from donors of various ages (8 to 19) months also produced a significantly high percentage of hapten-augmentable PFC. Experiments with B cells and thymus cells from 2-or 8-month old donors parked in lethally irradiated 2-or 8-months old recipients for 7 days revealed that neither lymphocytes from old donors or old recipients were capable of inducing the appearance of anti-Id-blocked, hapten-augmentable PFC in the lymphocyte population from 2-month-old donors. Thus, the results of this study indicate syner-gistic co-operation between B lymphocytes and thymus cells from old donors for the production of auto-anti-idiotypic antibody regulation with age. This production of auto-anti-Id antibody with age seems not to be an induced maturation event but perhaps an intrinsic property unique to lymphocytes from old donors.