Abstract: A method for testing petroleum or water wells. A tool is secured to a tubing and run into the well and to the reservoir zone to be explored. An upper and a lower seal isolate the reservoir zone. A piston is run into the tubing and pumped down to the tool. When sampling is to take place, a valve is opened and the reservoir fluid, together with drilling fluid filling the well, is allowed to enter the tool while displacing the piston. At the same time, measuring is carried out to identify a tracer added to the drilling fluid. When the tracer no longer can be traced, the drilling fluid contaminated reservoir fluid is displaced out of the tool by the piston and clean reservoir fluid is allowed to enter the tool. The invention also relates to an apparatus for practising the method.