The direct examination of tissue and biomaterials from prosthesis-related infections of twenty-five patients showed that the causative bacteria grew in glycocalyx-enclosed biofilms that were adherent to surfaces of biomaterials and tissues in 76 per cent. This high rate of recovery of adherent biofilm-mediated growth suggests that the process occurs commonly in the presence of a foreign body or biomaterial-related infection. Because of the adherent mode of growth of the infecting organisms, accurate microbiological sampling was difficult. The analysis of joint fluids or of swabs of excised tissue and of prosthetic surfaces often yielded only one species from what was a polymicrobial population based on electron microscopic studies. We adapted direct quantitative sampling methods from environmental microbiology in order to recover a large number of species from these infections, but comparison of the organisms isolated by these techniques with the morphological types that were seen by electron microscopy indicated that in some instances all bacterial components of the biofilms were still not being recovered.