Ticks, unique hematophagous arthropods and very effective vectors of a diverse spectrum of pathogens, possess an astounding array of salivary molecules that ensure their unnoticed and prolonged attachment to the host skin by counteracting host defences, including immune cells. Natural killer (NK) cells, belonging to the main cellular components of innate immunity, are specialized lymphocytes with the ability to recognize and eliminate pathogen-infected cells and tumour cells. Previously we showed that salivary gland extracts (SGE) derived from adult Dermacentor reticulatus ticks suppressed cytotoxic activity of human NK cells. The decrease was observed with SGE from both partially fed female and male ticks as well as from partially fed female Amblyomma variegatum and Haemaphysalis inermis. In this study, we determined that SGE from both sexes of adult D. reticulatus and A. variegatum and from females of Haemaphysalis concina inhibited cytotoxic activity of mouse NK cells in vivo stimulated with polyinosinic-polycytidylic acid (poly I:C). The decrease effect was observed with SGE from partially fed ticks, but no significant inhibition was determined with SGE from unfed ticks. On the other hand, no significant decrease of NK activity was detected after treatment with SGE from partially fed adult Ixodes ricinus, H. inermis, Rhipicephalus appendiculatus and Rhipicephalus pulchellus. According to the results from proteomic approaches we assume that the anti-NK activity molecule(s) is (are) very basic protein(s) with a molecular weight of approximately 30 kDa.