Harlequins (6) 18
Tries: Purdham 2, Sharp Goals: Orr, Purdham 2
Bradford (36) 42
Tries: Halley 3, Burgess, Menzies, Worrincy, Godwin Goals: Deacon 7
Halley scored three tries in the opening 21 minutes
Super League's third-bottom side Bradford swept to a seven-try win to dent Harlequins' play-off hopes.
Two tries in the opening six minutes from Bradford's Dave Halley set the tone before Robert Purdham hit back for Quins with Danny Orr adding the extras.
Halley, Michael Worrincy, Sam Burgess, Wayne Godwin and Steve Menzies added tries as Bradford cruised before Purdham and Will Sharp's late response.
Bradford's Paul Deacon capped the win converting seven faultless kicks.
Just six days after being beaten by Quins at home, the struggling Bulls turned the tables against the Super League play-off hopefuls.
Bradford began brightly and were rewarded when Chris Nero shrugged off the challenge of Orr to send Halley in unopposed.
Seconds later Deacon broke the defensive line for full-back Halley to canter over again.
The hosts responded when Luke Dorn ran a fine line to allow captain Purdham to drive over but it wasn't to be the start of a classic comeback.
Instead relentless Bradford added a third try as Deacon's kick through deflected off Orr into the hands of Halley for his hat-trick try.
Burgess picked up a loose kick to add another before first-half scores from Menzies and Michael put the result beyond doubt.
Godwin added the seventh try over from close range, which should ease the pressure on Bradford coach Steve McNamara.
Quins skipper Purdham and Sharp did cross for tries late on, but they were merely consolations.
Harlequins coach Brian McDermott:
"We will not make the play-offs if we play like that again, and quite frankly we will not deserve to make the play-offs playing like that.
"I am extremely concerned that we have a group of players who went out there and looked like they had never played together before.
"I am frustrated with myself as much as the players. There are questions that need to be asked about everyone at the club - myself included."
Bradford coach Steve McNamara:
"We really started strongly and built up a pretty impressive lead. We put ourselves on the front foot and we never really looked back.
"We played with far less anxiety and there were many more scoring chances that we did not take advantage of.
"The less said about the second half the better - it was not a great advert for rugby league but by then the game was won and I am happy we did such a professional job."
Harlequins: Melling, Wells, Gafa, Purdham, Sharp, Dorn, Orr, McCarthy-Scarsbrook, Randall, Ward, Esders, Golden, Williamson.
Replacements: Heckenberg, Haggerty, Gale, Bryan.
Bradford: Halley, Sheriffe, Platt, Nero, Tadulala, Sykes, Deacon, Lynch, Newton, Scruton, Langley, Morrison, Menzies.
Replacements: Burgess, Worrincy, Kopczak, Godwin.
Referee: I Smith (Oldham)