Poultry mites is one of the most important parasites in intensive poultry farming. There are many different types of mites, but the most common one is red poultry mite (Dermanyssus gallinae). This mites' bite indirectly puts animal health in danger by causing stress, pain, skin irritations and production decline, but it can also can also endanger the animals directly as a disease carrier. Many different acaricides have been used in order to suppress mite invasions, but they haven't been proven successful. Therefore a new drug, fluralaner (Exzolt, MSD) is being used. Fluralaner works as an inhibitor of the arthropodes' nerve system with an antagonistic action on GABA receptors and glutamate receptors and also stops egg production in females. The purpose of this study was to monitor the total number of leukocytes, the relative and apsolute number of individual lineages and the heterophile and eozinophile (H/L ratio) in laying hens kept in free range aviary systems. The terapy began at 38 weeks of age and the drug was administred according to the manifacturer's instructions, twice, seven days apart in a dose of 0,5 mg fluralaner per 1 kilogram of bodyweight orally via drinking water. The blood was sampled by jugular venipuncture before the therapy (day 0.) and then 7., 14., 21. and 60. day after treatment. The results have shown a decrease in the number of leukocytes, lymphocytes, heterophiles and eozinophiles, the number of monocytes has slightly increased, while the number of basophiles hasn't significantly changed. The changes in the H/L ratio indicate on a potential germ activation on the beginning of therapy and in later stages of therapy on the recovery. The laying hens have fully recovered from the mite invasions and hematological paramethers have been returned to their normal values.