The research is dedicated to the operative work of law enforcement agencies in terms of the use of undercover agents for conducting operative-search operations in the criminal environment for searching for the criminal activity of previously unknown persons (unknown accomplices). A comprehensive analysis of the main provisions of the Ukrainian legislation and fundamental international acts that regulate the use of undercover activities in Ukraine was conducted chronologically. It has been established that the European standards defining the limits of legal behaviour of undercover agents in the criminal environment are mainly based on the provisions of fundamental international acts on the protection of human rights and freedoms adopted in the 1948−1980s; the Ukrainian legislation regarding the issue under research began to be formed in 1990s; only since 2006, the Ukrainian justice system began to actively recognise the European judicial practice when passing verdicts in the field under study. It has been emphasised that the effectiveness of legal regulation of the use of undercover activities by law enforcement agencies depends on the ability to combine and direct positive factors accompanying the legal norm and block those that hinder it. The disparity of legal acts that fragmentarily regulate legal foundations of undercover activities negatively affects the efficiency of using the obtained results for combating crime. It is suggested to solve this issue through the unification of the provisions of the legal acts in order to achieve uniformity in the use of undercover methods by law enforcement agencies for combating crime according to the European legal standards. Taking into account a high crime rate in modern Ukraine, it is quite challenging to comply with or implement the latter in order to avoid incitement or entrapment to the extent of the regulation of lawful behaviour of undercover agents in the criminal environment, since solely passive behaviour of undercover agents not only fails to contribute to detecting criminal activities and documenting criminal intents of accomplices but rather exposes the undercover agents to the criminal world, which poses a threat to the life and health of both undercover agents and their relatives. In order to improve the effectiveness of crimefighting activities in Ukraine, this research justifies the legal regulation of the general principles of permissible lawful behaviour of undercover agents as regards empowering them to use active forms of denunciating the criminal activities of people in the criminal environment