An apparatus for the filling of a medical filter has at least one first space and one second space which are semipermeably separated by one or more membranes, with the spaces each having at least one inflow and outflow. The apparatus includes a detection unit having a first element and a second element, with a liquid discharge from at least one of the first and second spaces being detectable by the first element, and the state of the filling of the filter being determinable by the second element with reference to the detected liquid discharge. A method of filling a medical filter employs the apparatus.