Under a cooperative agreement, the Bureau of Mines, Magma Copper Company, and Asarco, Inc., tested backfilled ground support systems at two elevations in Magma's San Manuel, Arizona, Mine. The tests were mainly for guidance in designing the loading crosscuts of the new Asarco Sacaton underground mine. Two-piece, wide-flange steel sets for 8- by 8-ft (2.44- By 2.44-M) and 11- by 11-ft (3.36- By 3.36-M) openings were designed. Set spacing was 3 ft (0.91 M) and 5 ft (1.52 M), respectively. After erection, the sets were lagged with various test materials. The void between the lining and the ground was backfilled with sand and pea gravel. Instruments measuring strain (used to determine changes in stress) were placed beneath the sets and in the backfill. Results indicate the backfilled sets in the smaller drift (2315 grizzly level, panels 6 and 7) of 8- by 8-ft (2.44- By 2.44-M) cross section are a viable permanent support system, lower in cost than the standard formed concrete. Results are inconclusive in 2615 grizzly level, panels 5 and 6, where wide- flange steel sets were used with 5-ft (1.52-M) long concrete gob lagging of t-design.