The effect of increasing age on various tests of cell-mediated immunity was investigated in BALB/c mice both in vitro and in vivo with four different assay systems. The following results were obtained. 1) In contact sensitivity to DNFB, old mice (age 60 to 80 weeks) showed no differences in sensitization when compared to young adult mice (age 8 to 12 weeks). (In contrast, old NZB/W mice showed impaired contact sensitization when compared with young NZB/W MICE.)2) Unlike the reaction in contact sensitivity, cells from old BALB/c mice were defective in eliciting a graft-vs-host reaction. This was true also when a partially purified population of T cells was transferred. 3) In the mixed lymphocyte reaction, cells from old mice were as efficient or better than cells from young adult BALB/c mice in responding to or stimulating allogeneic cells. 4) Responses to PHA and Con A (Both T cell mitogens) were greatly reduced when old cells were cultured as compared with cells from young adult mice. Thus, we have found that within the same batch of mice, increasing age was associated with increased capabilities in some measures of cell-mediated immunologic function and decreased capabilities in other measures of the same.