Lymphocyte proliferation in dementia of the Alzheimer type (DAT) was evaluated by the level of mitogen response using phytohemagglutinin, pokeweed mitogen, concanavalin A and staphylococcal protein A. Lymphocyte subpopulations in the peripheral blood were also investigated using a monoclonal antibody with a fluorescence-activated cell sorter. The mitogen response in DAT was not significantly different from that in the normal controls. Lymphocyte subpopulations (Pan T, helper T, suppressor T, B cell, HLA-DR positive cell and helper/suppressor ratio) in DAT were not significantly different from those in the normal controls. These results suggest that the lymphocyte proliferative response to mitogens and the subpopulations in DAT may be natural in the normal aging process.