Lamb's 56, which followed his unbeaten century in the previous match against Glamorgan, was one of the highlights of a slow, rain-interrupted day, which lost 38 overs to the weather.
Northants, second in the table, were given a solid foundation by Rob Bailey (40) and Alan Fordham (39) before Lamb took over to take his side to 229 for 4.
Allan Warner, Derbyshire's fast bowler, produced a superb spell of 4 for 21 in 10 overs after tea as Gloucestershire slumped to 202 for 7 after an opening stand of 81 between Dean Hodgson and Bill Athey. Warner, so often overshadowed by Ian Bishop and Devon Malcolm, moved the ball around consistently to claim his best figures of the season.
Courtney Walsh, the Gloucestershire fast bowler who heads the national bowling averages, is to take a week off cricket. The West Indian strike bowler, who has claimed 41 victims so far, is suffering from a flu-type virus.Reuse content