Cyclic adenosine monophosphate (cAMP) levels of isolated human peripheral blood lymphocytes show age associated changes. These changes are apparent in basal cAMP levels as well as in cAMP levels after trypsin treatment. Both cord blood lymphocytes and lymphocytes isolated from the blood of old people exhibit lower basal levels of cAMP and diminished increase in cAMP by trypsin treatment, in comparison to lymphocytes isolated from the other age groups. The results seem to indicate a low number of reactive cells and a low reactivity of the cells in the case of cord blood lymphocytes, and a decrease in number of the reactive cells without a decrease in specific reactivity of isolated lymphocytes of old people.