This Bureau of Mines report describes an investigation of factors influencing economic viability of a proposed system to mine and process manganese nodules. The system consists of hydraulic dredging and ammonia leach processing designed to recover three metals: nickel, copper, and cobalt. A ferromanganese recovery plant is a considered option. Annual capacity is 3 million dry metric tons nodules. Aftertax rates of return (ror's) of only 7.38 and 6.63 pct are predicted for three- and four-metal base scenarios, respectively. Sensitivity analyses indicate that variations in commodity prices, metal recoveries, and deposit grades produce similar incremental changes in ror. Estimated cumulative effects of price variations are greatest. While three-metal operations generally yield higher predicted returns, potential increases in manganese price and process recovery could make four-metal operations most profitable. Maximum variations in both capital costs and three-metal operating costs result in ror changes of about 25 pct. In contrast, four-metal ror's exhibit variations to 70 pct. Even so only a best case analysis, utilizing favorable variations in all categories tested, generated ror's approaching a level thought necessary to trigger company interest. Consequently, it is concluded that nodule mining will not take place in the foreseeable future without significant financial incentives.