The distribution of T cells, B cells and murine senile amyloid protein in Peyer's Patches was examined in senescence accelerated (SAM-P/1) and control (SAM-R/1) mice ranging in age from two to ten months. An immunohistochemical detection of lymphocyte surface antigens of T cells and B cells. A murine senile amyloid protein in Peyer's patches was detected by immunohistochemical staining with the specific antiserum. Congo red staining and electron microscopy. The T cell population increased and B cell population decreased slightly with age in SAM-P/1 mice, but little change was observed with age in SAM-R/1 mice. Murine senile amyloid protein positive spots were seen surrounding the vessels of the thymus-dependent area at 6 months of age, and were observed throughout the Peyer's patches at 10 months of age, but were not observed by 10 months of age in SAM-R/1 mice.