Abstract. The aim of this paper is to investigate the effects of agricultural utilization of biodiesel byproducts. The hydrophilous by-products of transesterification, such as proteins, carbohydrates, minerals, vitamins, potassium hydroxide used as catalyser and methanol are concentrated in the phase of glycerol during biodiesel production. Agriculture can utilize these components but some effects of glycerol in soil do not serve the needs of plants. Several studies have investigated the different effects of glycerol in soil. The specific focus of our research is to analyse the relationship between seed germination and glycerol content of soil. During this research the inhibitor effect of glycerol, biodiesel by-product, and methanol on seed germination of ryegrass (Lolium perenne L.) and oilseed rape (Brassica napus) were investigated. In the case of some treatments, a different percentage of glycerol and methanol was applied, in order to establish the methanol content of soil that can balance the inhibitor effect of glycerol. Based on the obtained information, pot experiment was established with ryegrass to examine the effects on plant growth. Our research has not only studied the impact of biomass production, but variations in the rate of plant growth as an effect of different treatments were observed as well.