The backfilling of underground mines is a well known, if not commonly practised, method for disposing of mine waste while providing structural support. In recent years, increasing environmental pressure against surface methods of waste disposal and concern about subsidence have resulted in an increase in the popularity of infilling of mine adits. For example, backfilling of abandoned mines in the U.K. is seen as a means of reducing the possibilities of subsidence, thereby opening up 'under-mined' areas for potential development. This apparently ideal solution for the disposal of mine wastes is now being questioned in the U.S. on environmental grounds. The concern is that the wastes might be geochemically unstable and lead to contamination of the soil and groundwater. This is particularly the case when the infill being used has a high metal content. This article discusses research that was carried out by the U.S. Bureau of Mines in response to the Underground Injection Control Regulatory Programme. This regulatory programme, which encompasses the bulk infilling of underground mines, was set up by the Environmental Protection Agency under the auspices of the Safe Drinking Water Act (1974).