The quality of drinking water consumed by end users largely depends on the technical condition of internal water supply installations.The aim of the study was to assess the impact of factors such as the technical state of the installation and the type of materials used on the growth and abundance of microorganisms. The research was conducted on water samples collected from internal water supply systems. The analyses involved quantification of the total number of bacteria at 22 and 36 degrees Celsius. The obtained results indicated a correlation between the type of material (e.g. plastics, copper) and the technical condition of the installation, and the occurrence of specific microbial groups. The study highlights the importance of regular monitoring of the condition of internal plumbing systems and the appropriate selection of construction materials to ensure high microbiological quality of domestic water and microbiological safety.The findings are also of practical importance, as they may contribute to the development of recommendations for the design, construction, and operation of water supply installations that minimize the risk of water contamination.