The Bureau of Mines evaluated the impact of the depletion allowance upon the economic availability of copper from domestic deposits. The potential long-run percentage depletion deduction, cumulated over the life of the properties, was estimated at approx. $2.5 billion for producing properties and up to $11.7 billion for nonproducers. Higher federal income tax payments if the allowance were repealed, were estimated at $1.5 billion for producing properties & up to $8 billion for nonproducers. The percentage depletion allowance, through its role in determining federal income tax liability, was found to be very significant in affecting profitability. Repeal of the allowance could result in major domestic copper availability reductions at all price levels, now & in the future, as a result of reduced profit margins for current producers, & postponement of development decisions for nonproducers. A 50-percent reduction in the percentage depletion rate applicable to copper would result in a significant decrease in profitability for nonproducing properties. For producing properties, the percentage depletion rate on copper was shown to be less significant than the income limitation that is placed upon the total amount of the depletion deduction. Repeal of the minimum federal tax provision was shown to be significant in enhancing profitability for both producing & nonproducing deposits. The percentage depletion allowance operates as an important method of capital recovery & production incentive.