A system for holding sample tubes used in immunoprecipitation and similar laboratory techniques. A rack comprises top and bottom plates spaced apart from each other and defining rows of holes to receive the sample tubes and hold the sample tubes in a pair of spaced-apart rows. A magnet holder is configured to slide between the top and bottom plates and between the two parallel rows of sample tubes such that when the magnet holder is fully inserted between the rows of sample tubes, magnets held by the magnet holder align with the sample tubes in the two parallel rows.