A statistical relationship between the rate of ferric ion production by a strain of Thiobacillus ferrooxidans and various levels of cell concentration, Fe concentration, Na concentration, and temperature was studied by a direct colorimetric method at 304 nm. The relationship was linear (90 to 93%), cross-product (3 to 4%), and quadratic (1 to 2%). The levels of cell concentration and Fe concentration and their respective interactions with one another and the other factors had the most significant effects on the regression models. The solution of the quadratic response surface for optimum oxidation was a saddle point, and the predicted critical levels of temperature, cell concentration, Fe concentration, and Na concentration ranged between -6 and 2 degrees C, 0.43 and 0.62 mg/ml, 72 and 233 mM, and 29.6 mM, respectively.