The study presented concerned an influence of an application of fish oil from cod liver with marine microalgae in feed doses of dairy cows in the first period of lactation on selected biochemical parameters of blood and milk yield. The fish oil with microalgae was applied to cows from 7^th day of lactation for a period of 8 weeks, in an amount of 1% of feed dose dry matter. The additives used caused decrease in feed intake, what in turn resulted in lowered milk yield. Also a reduction of fat and protein content in milk was observed. Increased PUFA content in feed dose caused an increase in triglycerides, cholesterol and β-hydroxybutyrate acid content in blood serum. An increase in total bilirubin concentration was also noted, however enzymes activity did not point any functional liver disorders.

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