This investigation has been carried out for the purpose of determining the effect of solid particle size distribution on the roasting and leaching of a complex sulfide ore. The effect of pulp density in the leaching process has also been studied. The oxidizing roast of the combined sulfide concentrates, prepared by bulk sulfide flotation and tabling, was conducted with a fluidized-bed kiln. The calcines were leached by dilute sulfuric acid in an autoclave. The following conclusions have been reached: 1. In the fluidized-bed roasting, the solid particles within the size range of minus 35 to plus 150-mesh would have a higher overflow rate from the fluidized bed. This inference might be effective only when the roasting is conducted under the conditions performed in this investigation. 2. In the process combining fluidized-bed roasting and dilute sulfuric acid leaching, the effect of solid particle size is significant. The solid particles within the size range of minus 65 to plus 150-mesh have the highest iron recovery, the highest copper and zinc extraction rates and the lowest sulfur content of leach residue. 3. In dilute sulfuric acid leaching there is no significant effect of pulp density within the range tested in this investigation. The general trends are: iron recovery decreases as the pulp density decreases, extraction rates of copper and zinc decrease as pulp density increases, and there is little pulp density effect on the sulfur content of leach residue. / Master of Science