The titer of thymic serum factor was measured in adult and old CBA females 15, 30, and 60 min, 24, 48, 72, and 96 h after injection of 3% suspension of sheep erythrocytes and changes of this parameter under the effect of epithalamin were studied in old immunized mice. The titer of thymic serum factor increased appreciably in adult mice virtually at all periods of the study after immunization (a drop was observed only 72 h after immunization). In old mice the titer of thymic serum factor virtually did not change after injection of sheep erythrocytes, while immunization of old mice preinjected with epithalamin significantly increased this parameter. Not only the values, but their dynamics in old mice injected with epithalamin corresponded to those in immunized adult animals.