Knowing the significance that a factor has in influencing a measured response is paramount for optimisation and cost control. The significant factors can be optimised while the insignificant ones can be set at levels where the least cost is incurred. The purpose of this study was to investigate the factors that influence the dissolution of nickel laterites using a mixed culture of chemolithotrophic bacteria (Acidithiobacillus ferrooxidans, Acidithiobacillus caldus and Leptospirillum ferrooxidans). The significance of each factor and their interactive effects were evaluated using statistical design of experiments quarter fractional factorial designs 2(III)(5-2) and dissolved nickel was taken as the measured response. The factors under this study included pulp density, particle size, pH, size of bacterial inoculum and substrate type. Results obtained indicated that inoculum size was not statistically significant while the rest of the factors were statistically significant. Under the ranges studied the interaction between the variables was found to be statistically insignificant, apart from that between pulp density and type of substrate. The results also showed that recovery was maximized at low pH and low pulp density. In the range studied, particles of less than 38 pm had a negative influence on nickel recovery. Sulphur substrate also showed better effects than pyrite. (C) 2008 Elsevier B.V. All rights reserved.