The surface components of bioleaching microorganisms such as Thiobacillus ferrooxidans apparently play an essential role in the adhesion of the bacteria to minerals. These components are synthesized in different amounts depending on several factors, such as the type of energy source and the lack of some nutrients. We have prepared antibodies against both whole cells and outer membrane preparations from T. ferrooxidans grown in ferrous-iron or elemental sulfur. The antibodies prepared were specific for T. ferrooxidans cells and allowed us to discriminate between iron- and sulfur-grown cells. The antigens were analyzed by Western immunoblotting and their localization on the surface of the cells was accomplished by immunoelectron microscopy using gold-labeled protein A. Interestingly, the sulfur-grown and not the iron-grown cells showed very long surface appendages reacting with the antibodies against outer membrane preparations of sulfur-grown T. ferrooxidans . These structures were similar to flagella and may be very important in adhesion of the microorganisms to solid sulfur and eventually to other minerals.