The objective of our study was to determine changes in protein and monosaccharide content in select grass species due to an application of the kelp species Ecklonia maxima extract. The field experiment was arranged as a randomized sub-block design (split-split-plot) with three replicates. The following factors were examined: growth stimulant with the trade name Kelpak SL, which was either applied at the rate of 2,000 dm3·ha-1 or not applied, pure-sown grass species, and cultivars (Dactylis glomerata, cv. Amila and Tukan; Festulolium braunii, cv. Felopa and Agula). Protein compounds and monosaccharides were determined in the dry matter of the tested grasses. The biostimulant Kelpak significantly increased both protein compounds and monosaccharides as well as the carbohydrate-protein ratio in the plants. The grass species and their cultivars had different levels of protein compounds and carbohydrates.