Bailey reached lunch on an unbeaten 78 as Hampshire made 114 for two against Leicestershire at the Rose Bowl.
They had been reduced to nought for two as openers Jimmy Adams and Michael Carberry both fell for ducks. But Bailey hit 12 boundaries in 105 balls in his century stand with Liam Dawson (34no).
Promotion favourites Lancashire had Worcestershire 72 for two after being put in at Old Trafford. Wayne White and Luke Procter had a wicket each, with Moeen Ali (20no) and former Sri Lanka batsman Thilan Samaraweera (18no) at the crease.
At Sophia Gardens, Northamptonshire took regular wickets as Glamorgan made 74 for four.
Stewart Walters' 23 was the home side's highest score, while Steven Crook claimed two for 14 for Northamptonshire.
Meanwhile, Ravi Bopara took a wicket with the final ball of the session to leave Gloucestershire 71 for three against Essex.
David Masters and wicketkeeper James Foster combined for the first two wickets before England all-rounder Bopara trapped Alex Gidman lbw.
Dan Housego will resume on 33.
- Sports & Recreation
- Liam Dawson
- Michael Carberry