This publication presents the principles of organization, implementation, assessment and exploitation of proficiency testing results in the work environmental laboratory based on basic requirements included in standard PN-EN ISO/IEC 17043 and ISO 13528. The basis for the proper functioning of the laboratory is to use and observe the basic requirements for the competence to carry out the tests and the guidelines on ensuring reliable and accurate results, specified e.g., according to the guidelines described in the standard PN-EN ISO/IEC 17043. The confirmation of the laboratory competence is the obtained accreditation. To obtain this certificate several conditions, including proficiency testing (PT) should be met. The main aspects of this paper is to show the role of proficiency testing in the process of assuring a properly functioning quality system in the laboratory. The accreditation requirements, the types of proficiency testing schemes, methods of statistical analysis and interpretation of results are also discussed by the authors. Med Pr 2016;67(2):267–283