The research was undertaken to determine the effect of the biostimulant Kelpak SL, derived from brown seaweed species Ecklonia maxima (kelp), on structural and non-structural carbohydrates, as well as lignin content in orchard grass and Braun’s festulolium. The experiment was a split-plot arrangement with three replicates. It was set up at the experimental facility of the University of Natural Sciences and Humanities, Siedlce, in late April 2009. The following factors were examined: an application of the plant growth regulator Kelpak SL applied at the rate of 2 dm³· ha^-1 vs an untreated control (0 dm³· ha^-1), pure sown grass species and cultivars grown in monoculture: Dactylis glomerata, cv. Amila and Tukan, as well as Festulolium braunii cv. Felopa and Agula. This study revealed that an application of Kelpak significantly reduced cellulose, hemicellulose and lignin contents of the grasses but significantly increased non-structural carbohydrates, regardless of the remaining factors. Non-structural carbohydrates were the highest in Kelpak-treated Festulolium braunii (on average, 232.7 g · kg^-1).