The invention relates to a drill bit assembly for fluid-operated percussion drill tools comprising a percussion bit (1) having a head portion (3) formed with an axially extending stub shank (32). The stub shank is provided with axially extending splines (36), which are slideably engageable with complementary splines (35) formed on a drive chuck (4). Rotational drive from the chuck (4) may be transmitted to the stub shank (32) by means of the splines. Bit retaining means (6, 7, 41, 42) at the chuck are adapted for engagement with complementary retaining means (37, 51) at a spline portion of the stub shank to retain the stub shank in the drill bit assembly. Engagement means on the chuck (4) are adapted for connecting the chuck (4) to a drive means (5) of the fluid-operated percussion drill tool. The invention also relates to a down-the-hole hammer comprising an external cylindrical outer wear sleeve (5), a sliding piston (8) mounted for reciprocating movement within the outer wear sleeve (5) to strike a percussion bit (1) of a drill bit assembly located at the forward end of the outer wear sleeve (5) wherein the drill bit assembly is an assembly as described above.