Pathogenicity of methicillin-resistant (MR) strains of S. aureus is difficult to determine from clinical or epidemiological data. The degree of their pathogenicity has been tested in an in vitro phagocytic assay using isolated polymorphonuclear leukocytes (PMN), since PMN are considered to be the main defense mechanism against infections by S. aureus. In the phagocytic assay, survival of MetR strains was measured simultaneously with that of a methicillin-sensitive (MetS) control strain. After 30 and 60 min of incubation, MetR strains survived the phagocytic action of PMN better than the MetS strain. Amplification of resistance towards the antibiotic in three MetR strains did not modify their survival in the phagocytic assay. In conclusion, MetR strains have a definite pathogenic potential and this potential is not affected by the level of resistance towards methicillin.