A self-contained apparatus for concentrating and dialyzing protein and other macromolecular liquid samples rapidly to precise, predetermined microliter quantities. An elongate rod of predetermined volume, having irregular cross section along its length, is disposed within a semi-permeable membrane sample container in spaced juxtaposition with the membrane to maximize the rates of concentration and dialysis. The rod also cooperates with a sample collection chamber sealingly interconnected detachably with the bottom of the membrane to automatically provide a predetermined final sample volume precise to within two microliters. The rod includes a member, detachably coupled with and extending into the cllection chamber for providing the collection chamber with a predetermined volume.