The Bureau of Mines, through a cooperative agreement with Duquesne Light Company of Pennsylvania, field tested a method of using pneumatic stowing of crushed limestone to construct seals in long-abandoned mine openings that were part of an active mine closed shortly before sealing. A total of 13 openings at 11 sites were backfilled. In pneumatic stowing, material is conveyed through a pipeline and into the mine opening under low air pressure. The stowing equipment consists of a power supply, a blower, a feeder to inject material into the pipeline, and nozzle for directing the placement. Material is ejected from the nozzle at high velocity, creating a high- compaction fill upon impact. The fill material used was well-graded limestone aggregate up to 1 1/2 inches in diameter, with sufficient fines. This fill was modified at five locations by the addition of portland cement, expansive cement, and/or bentonite. The pneumatic stowing method is safer and faster than conventional methods, is cost competitive, and eliminates or reduces the exposure of workers to possible hazards since the nozzle can be kept outside the mine opening. The technique was used at remote locations where other methods could not be used without disturbing the environment.