Abstract: In a method of replacing a section of an old cast iron main with a new main of plastics pipe material, the section to be replaced is isolated from the adjacent live main after a by-pass is fitted between the section and the main. One end of the pipe is then pushed up the isolated section until a desired length is located within the isolated section and the other end of the pipe is connected to the end of the adjacent main. This enables the isolated section to be kept alive solely by gas flow through the pipe from the main and the by-pass can therefore be removed.