Studies of levels of hemagglutinins (HA) and hemolysins (HL) with age are studied in hemolymph of the gastropod mollusc Achatina fulica and in response to immunization by sheep erythrocytes (SE). The animals of three age groups (1, 2, and 3 years) were studied. The HA and HL levels were evaluated before the immunization and in 3, 5, 10, and 15 days after the antigen stimulation. The HA titer was determined by hemagglutination reaction, the HL titer, spectrophotometrically by the release of hemoglobin from SE. It was established for the first time that with age a twofold increase of the HA concentration and a fourfold decrease of the HL concentration occurred in the hemolymph. The HA system in molluscs of all ages provides the early response to the antigens (the first 5 days). The lytic system in the 1- and 2-year old animals responds by a gradual rise of the HL level during the first 5 days after immunization and by a decrease of the concentration of these lytic factors in the course of development of the defensive reaction (the 5-15th days).