Munster were beaten by 18-13 by the Scarlets in Wales.
Gareth Davies and John Barclay scored a try each for the home side, with Munster's only try coming from winger Gerhard van den Heever.
That result meant that Leinster went top, after they edged their game with the Glasgow Warriors by 28-25 at the RDS.
Connacht also won, beating Treviso by 38-6.
Meanwhile, in today's Munster Junior League action;
Division 1; Muskerry v Kilfeacle & District, but Clonmel's game at Skibbereen is off.
Division 2; Clanwilliam v St Senan's, Newport v Thurles
Division 3; Abbeyfeale v Carrick-on-Suir
All those games kick off at 2:30.