Abstract: A protective arrangement or "junk bonnet" (30) for preventing debris entering a downhole tool assembly (14) employed in the drilling of a well. The junk bonnet (30) is in the form of an annular seal which fits around a tube (18) and inside a sleeve (24) which are relatively movable to operate a downhole tool assembly (14) of which they form part. The junk bonnet (30) is provided with sliding fluid seals (42, 44, 48, 50) which seal against the external surface (19) of the tube (18), and against the bore (25) of the sleeve (24). A longitudinal fluid passage (52) links opposite ends (54, 56) of the junk bonnet (30). The passage (52) is sealed in use by a plug (60) or other suitable valve. An annular volume ("C") below the junk bonnet (30), between the tube (18) and the sleeve (24), and above a seal (26) forming part of the downhole tool assembly (14) is filled through the passage (52) with hydraulic oil during setting-up and prior to closure of the passage (52) by the plug (60).