The effects of antibodies to calcium-binding S100B protein diluted to 10(-12) (LAS100B) on the long-term sensitization in the Helix lucorum snail (neurobiological model of the anxious-depressive state) were evaluated. The administration of LAS100B prior to conditioning of long-term sensitization in the terrestrial snail 10 min prior to the first electric stimulus) prevents strengthening of the defensive reaction of withdrawing the ommatophores (eye tentacles) and the defensive reaction of closing the pneumostome. This effect is termed "protective", as it prevents the conditioning of long-term sensitization. At the same time, snails given an injection of saline developed long-term sensitization with a significant strengthening of the defensive reactions of withdrawing the ommatophores and closing the pneumostome. When LAS100B was administered before long-term sensitization in advance, the membrane and threshold potentials of premotor interneurons, which regulate defensive behaviour, decreased to a significantly lesser extent compared to the long-term sensitization arm. It is possible that the "protective" effect is linked to the mechanisms of maintaining the membrane potential and changes in extra- and intracellular balance of calcium-binding S100B protein.