Northamptonshire started the new County Championship campaign with a resilient batting effort to frustrate Middlesex.
Northants were wobbling on 83-3, but half-centuries from captain Alex Wakely (76) and wicketkeeper Adam Rossington (67) helped them to 310-6 at the close.
West Indies skipper Jason Holder is unbeaten on 36 on debut.
Tim Murtagh (4-43) did most of the damage with the ball, while paceman Toby Roland-Jones played his first Championship match since April 2018.
Roland-Jones, who won four Test caps for England in 2017, missed most of last season with a stress fracture in his back, but claimed the prize wicket of Rossington late in the day.