The present studies were carried out in order to explore, whether or not senile mice react in the same manner to a single injection of either 50 μg Serratia marcescens endotoxin (ET) or 3 × 109 killed Bordetella pertussis organisms (PO) as young adult mice. This was not the case. The following essential differences were observed. (1) Aged mice produce PO-mediated blood leukocytosis to a distinctly reduced extent, as compared to young adult mice. (2) The injection of either ET or PO into adult mice led to a short-lived, but significant increase in the numbers of cells producing 19S hemolysins to sheep erythrocytes, whilst the response regarding those cells synthesizing 19S antibodies to rabbit erythrocytes was very poor. Thereby, the reactivity of young adult mice was found to be very homogenous. In contrast to untreated young adult mice, the spleens of untreated senile animals contained on the average larger numbers of plaque-forming cells, the values differing strongly from spleen to spleen. Following treatment with either ET or PO the senile mice showed extreme differences regarding their capacity to react with an increase in hemolysin-forming spleen cells, thereby, reaching the maximum response in each case later than young adult mice. (3) There was no close positive relationship between the existence of cells producing 19S hemolysins to sheep erythrocytes and the occurrence of those synthesizing corresponding antibodies to rabbit erythrocytes. (4) Aged mice showed about a 10-fold increase to the lethal action of ET, as compared to young adult mice.