Surrey seamer Matt Dunn claimed 3-53, including England captain Alastair Cook, as Essex were restricted to 225-6 on day two at The Oval.
Looking to make up for lost time after the opening-day washout, a succession of Essex batsmen got out after making starts to leave the match evenly poised at the halfway point.
That included Cook, who helped Jaik Mickleburgh (38) put on 60 for the first wicket but was caught behind off a short ball for 39.
Dunn, the 21-year-old from Egham, later removed Tom Westley (31) and Greg Smith (0) in the same over, while Ravi Bopara reached 49 before becoming a rare wicket for part-timer Jason Roy (1-15).
Surrey's Zafar Ansari was taken to hospital after diving head-first into a boundary hoarding while attempting to save some runs in the field. Ansari was later discharged from hospital but no decision on his participation in the remainder of the match will be made until Tuesday.
Stephen Moore's 106 not out led Derbyshire to 203-2 in reply to Hampshire's first-innings 328 at Derby.
Moore, who joined from Lancashire during the winter, hit 14 fours and a six and put on an unbroken third-wicket stand of 93 with Shivnarine Chanderpaul (35no).
Hampshire had earlier added 97 runs to their overnight 231-5 with Joe Gatting (67) and Michael Bates (31) leading the way. Derbyshire left-arm seamer Mark Footitt finished with 3-71.
At Canterbury, Leicestershire's lower order came to the rescue as they reached 333-9 after being sent in by Kent when play finally got under way a day late.
Opener Matthew Boyce hit 68 but Leicestershire had slipped to 224-8 before Jigar Naik (41no) and Anthony Ireland (52) combined in a ninth-wicket partnership of 85.
Off-spinner Adam Riley (3-52), preferred to England international James Tredwell in the Kent team, made Ireland his third wicket shortly before stumps.
And Gloucestershire produced a solid team effort with the bat to take advantage after bowling out Glamorgan for 145 in Cardiff.
Opener Chris Dent (52), William Tavare (42), Hamish Marshall (32) and Tom Smith (32no) all contributed as Gloucestershire reached 230-9 at stumps, a lead of 85, despite the best efforts of veteran Glamorgan spinner Dean Cosker (5-46).
Graham Wagg had earlier been left stranded on 54 after leading the Glamorgan recovery from 47-6 at the start of the day. Gloucestershire seamers Will Gidman (3-34) and David Payne (3-34) each finished with three wickets.