Administrative barriers to social insurance program take-up are pervasive, including in subsidized health insurance. We conducted a randomized controlled trial with Massachusetts' Affordable Care Act marketplace to reduce these barriers and other behavioral frictions. We find that a “check the box” streamlined enrollment intervention raises enrollment by 10.5%, more than personalized reminder letters (7.6% increase) or generic reminder letters (4.3% increase). Effects are concentrated among individuals eligible for zero-premium plans, who faced no further administrative burdens of setting up payments. Producing this enrollment effect through premium reduction would cost about $7 million in subsidies, highlighting the importance of these burdens.