Abstract: Verticillium wilt caused by Verticillium spp. is difficult to suppress and results in severe yield losses in a broad range of crops including oilseed rape (OSR) and cauliflower. The objective of the study was to develop a sustainable seed treatment that will protect oilseed rape and Brassica vegetables from Verticillium wilt. Preselected Serratia and Paenibacillus isolates showing antagonistic properties against fungal pathogens were compared for their plant growth-promoting (PGP) potential under a range of plant growth conditions. Serratia treatment resulted in a variety of levels of PGP, while Paenibacillus strains damaged roots of the seedlings under gnotobiotic soil free conditions. The selected strains from each genus, S. plymuthica 3RP8 and P. polymyxa Sb3-1, were tested for their PGP potential under sterile and non-sterile soil conditions. P. polymyxa Sb3-1 did not have a significant effect on plant growth in non-sterile soil; however it did promote plant growth in sterile soil. S. plymuthica 3RP8 had a mild non-significant PGP effect on the seedlings in gnotobiotic soil-free conditions and in unsterile soil. Similarly to the P. polymyxa Sb3-1, HRO-C48 significantly increased the weight of the OSR seedlings in sterile soil. This indicated that the choice of growth environments is crucial for the investigation of plant-bacterium interaction. In order to study the biocontrol efficacy of P. polymyxa Sb3-1, it was applied to the seeds of OSR in two different concentrations. The resulting one week old seedlings were infested with V. longisporum EVL25 using a root dipping method. Both treatments induced a significant increase in the lengths of the seedlings, however no significant reduction of disease was observed. The treatments with S. plymuthica 3RP8 did not result in any significant improvement in the seedlings’ health. The field trials and screening for new antagonistic isolates are ongoing.