Ireland centre Gordon D'Arcy scored Leinster's only try to take them top of the Magners League with a hard-earned 13-8 victory against Ospreys at the Liberty Stadium.
Centre Gordon D'Arcy scored the Irish side's only try as they were forced to come back from 5-0 down at half-time. Felipe Contepomi registered Leinster's other points with two penalty goals.
Ospreys, minus a total of 16 internationals because of RBS 6 Nations squad calls and injury, earned some consolation in the ninth minute of injury time when Dan Biggar's drop-goal assured them of a losing bonus point.
Leinster's stay at the top might only be temporary with Munster playing Newport-Gwent Dragons at Rodney Parade tomorrow afternoon.
It took Sonny Parker's try in the 39th minute to break the deadlock in an uninspiring first-half.
Parker broke through three would-be tacklers, including Stan Wright and Cameron Jowitt, to cross with the try being awarded on the advice of the television match official.
That was one of only a handful of scoring chances in the first half which saw a young Ospreys side match themselves against a much more experienced Leinster.
But it was a first half of low quality. Poor kicking and some poor handling in wet conditions rendered the scoreboard redundant until the latter stages of the half.
Biggar was off target with the only penalty goal attempt of the half from 45 metres out.
And Leinster's only scoring chance was ruled out by the TMO after Ospreys prop Cai Griffiths had got underneath number 8 Ricky Elsom's attempt to cross the line.
That followed Shane Jennings' kick and chase ahead which was well defended by hooker Ed Shervington and replacement wing David Bishop.
But Ospreys had much the better of the last 15 minutes and deserved their interval lead.
Having failed to get on the scoreboard before half-time it took the Irishmen just two minutes of the second half to cut the Ospreys lead to two points with a Contepomi penalty goal.
And Contepomi put Leinster in the lead after 54 minutes with a second penalty after Ospreys number eight Filo Tiatia was sin-binned for killing the ball at a ruck on his own line.
With the extra man Leinster's greater experience began to tell as they scored a try through D'Arcy after a break by Contepomi into the 22 as the Ospreys' defence was called into question.
Contepomi followed Tiatia into the sin-bin for a professional foul as the Ospreys tried to make a dent in the eight-point deficit.
Ospreys had a chance of grabbing a losing bonus point in injury time but Jonny Vaughton's stray pass did not find Tiatia unmarked on the left touchline.
The home side's efforts were rewarded when Biggar fired over a drop-goal - the last kick of the match - from in front of the posts.
England Under-20s head coach Martin Haag was full of praise for his World Rugby Under-20 Championship-winning side after Harry Mallinder inspired them to a 45-21 win over their Irish counterparts and a third title in four years.