The mechanisms responsible for the development of a pyogenic infection (most commonly due to staphylococci) in the vicinity of an implanted foreign body have been studied recently by several investigators. Thus, we have been able to demonstrate that the phagocytic function of residential polymorphonuclear leukocytes (PMN) is deficient in the presence of a foreign body. Others have shown that in the presence of foreign surfaces, microorganisms produce extracellular amorphous material, the pathogenic role of which is still to be defined. In the present study we use a novel assay system to demonstrate that Staphylococcus aureus Wood 46, after attachment to polymethylmethacrylate (PMMA), shows increased resistance to the phagocytic-bactericidal action of normal PMN. The first step of this assay involves the reproducible attachment of [3H]thymidine-labeled bacteria to PMMA cover slips. During the second step, attached bacteria were exposed to guinea pig peritoneal exudate PMN. In the third and final step, attached S. aureus cells were removed from the cover slips using a procedure harmless to the bacteria. The extent of bacterial detachment was estimated by radioactive counts and their viability by standard colony counts. Whereas bacteria that were attached artificially and rapidly by centrifugation and immediately exposed to PMN were killed in the phagocytic assay, bacteria adhering spontaneously to the cover slips for a prolonged period of time were more resistant to the killing action of the phagocytes. The spontaneous adherence of S. aureus to PMMA renders it poorly susceptible to the killing action of PMN.