Abstract: In the representative embodiment of the new and improved apparatus for testing earth formations disclosed herein, fluid-admitting means are sealingly engaged with a potentially-producible formation and a selectively-movable chamber is expanded to draw mudcake and other plugging materials from the isolated face of the formation into the receiving chamber. Thereafter, the chamber is shifted to communicate the screened entry port of the fluid-admitting means with the isolated formation so as to retard or prevent the erosion of loose formation materials as the formation is tested. When the testing is completed, the chamber is cleared and returned to its initial port-closing position and the fluid-admitting means disengaged to ready the apparatus for subsequent operations.