A trade union is an independent and democratic organization of workers, joined voluntarily to represent, promote, improve and protect employees' professional, work-related, economic, social, cultural and other individual and collective interests. In this paper, the author analyses the role of trade unions in the procedure of cancellation of an employment contract at the initiative of the employer. In particular, the analysis focuses on the procedure, the scope and the legal consequences of delivering an opinion of a trade union on the warning to an employee on the existence of legal grounds for dismissal. In positive labour legislation of the Republic of Serbia, an employee may seek an opinion of a trade union that he/she is a member of, along with a response to a warning issued by the employer on the legal grounds for the cancellation of an employment contract under conditions prescribed by law, within the time period specified for the employee's response to the warning. These regulations apply only in case of dismissing an employee who is a trade union member; therefore, they are not mandatory if an employee is not a trade union member. These regulations provide for an uneqaul position of the employees who are not members of a trade union in comparison to the employees who are members of a trade union. The employer is obliged to consider the delivered opinion in terms of legal norms. Yet, according to the interpretation of the Supreme Court of Cassation, an opinion of a trade union is not binding for the employer. This further indicates a minor role of trade unions in the protection of employees. An employee may procure and have the opinion of a trade union or the employees' representative delivered only regarding a warning on the existence of legal grounds for dismissal, but not with regard to the decision on termination of an employment contract.