Because multiple myeloma is a malignancy that appears in aging humans, the murine plasmacytoma model was selected to investigate the association between tumor formation, age, and immunity. Plasmacytomas were induced in young, middle-aged, and old BALB/c mice by intraperitoneal injections of pristane. Old mice were more vulnerable than middle-aged mice who were more vulnerable than young mice to histopathologically documented plasmacytoma formation. The difference between old and young mice was significant (p less than .05). The increased frequency of plasmacytomas was associated with decreased in vitro immune parameters in pristane-treated mice compared with age-matched saline-treated controls. The decrease was especially marked with T cell responses (mitogenic responses to concanavalin A, phytohemagglutinin, or allogeneic spleen cells) and natural killer (NK) cell activity. It was less striking with the predominantly B-cell parameters of mitogenic response to lipopolysaccharide (LPS) and LPS-induced plaque-forming cells. These findings support the hypothesis that old mice have increased vulnerability to plasmacytoma formation because of diminished NK and T cell activities.