The interaction of galena and sphalerite minerals with the metabolite obtained from Bacillus polymyxa has been examined through adsorption, electrokinetic, microflotation and flocculation tests. The adsorption density of the carbohydrate component of the metabolite for sphalerite exhibits a characteristic maximum in the pH range of 6-7, while in the case of galena the amount adsorbed increases with increase of pH. On the other hand, the adsorption density of the bacterial protein shows a continuous decrease with increase of pH, for both the minerals. The adsorption affinity of both the metabolic components is higher for galena vis-A-vis sphalerite. The electrophoretic mobility of the chosen minerals becomes less negative after interaction with the metabolite, in proportion with the time of interaction. Interestingly, the isoelectric point of sphalerite is shifted to less acidic values after treatment with the metabolite, but that of galena is unaltered. Bioflotation and bioflocculation studies on a synthetic mixture of galena and sphalerite demonstrate that galena can be selectively depressed or flocculated from sphalerite under appropriate conditions. Co-precipitation tests confirm complexation of lead and zinc species with the metabolic products, in the bulk solution. Possible mechanisms of interaction between the chosen sulphide minerals and the bioreagents are discussed. (C) 2003 Elsevier B.V. All rights reserved.