Abstract: The invention concerns a screening method and machine for size sorting in a fluid (9), into two categories, of materials with different volumes and density close to that of the fluid used after total immersion in said fluid. Mechanical stirring (6) means upstream of the sieves, optionally combined with the use of surfactants in an aqueous medium, maintain said materials to be size-sorted suspended in said fluid. The fluid circulates from upstream to downstream of the sieves. Only the materials below a certain size, volume or weight, maintained suspended in the fluid, pass through the size-sorting sieves (15). The sieves are immersed in the fluid and do not slow down the liquid flow passing through them. To obtain further size-sorted categories, it is possible to organise the inventive process sequentially. The invention is useful in poultry processing plants, processing feathers and down for padding purposes, purifying foreign matter in pasty mixtures and the like.