Effect of dehelminthization on milk yield and milk composition in dairy cows. The aim of this study was to assess the effectiveness of action of selected antiparasitic preparation on milk productivity of cows and milk composition. In coproscopic analyses the qualitative methods by Willis, Żarnowski and Josztowa were used, as well as the quantitative McMaster method. A preparation containing eprinomectin was used for dehelminthization. The study involved100 females after first lactation, which were divided into 2 groups 50 head each: experimental (D) - dewormed in May 2 weeks before going to pasture, control (K) – not dewormed. From the experimental group 15 animals were selected (D1) which were dewormed 2 days before parturition. Analogous group of cows (K1) was selected from the control group (at the same stage of lactation, but not dewormed). It was proved that eprinomectin has a positive effect on milk yield, fat level and dry matter in milk, whereas the effect of this preparation on protein content was not confirmed. It was found that the most favorable time of deworming cows is perinatal period.