It is established that the effect of thymus-derived species is connected with the cyclic nucleotide system. The action of thymus-derived immunocorrectors (thymalin, thymagen, vilosen) on catabolic processes of cyclic nucleotides has been observed under conditions of anaphylaxy and sensibilization. They show that sensibilization of the animal is bound up with a decrease of the cAMP/cGMP ratio. Anaphylaxis induces levelling of the cAMP/cGMP ratio up to the reference level. So, activity of enzymes of cyclic nucleotide catabolism grows due to the influence of thymogen, thymalin and vilosen in lymphocytes of sensibilized guinea pigs and tends to an increase in lymphocytes of anaphylaxis-treated animals.