In this paper we describe the in vitro interaction between three strains of methicillin-sensitive Staphylococcus aureus (MSSA) [NCTC 11561 and two strains derived from patients (PMSSA)] and endemic methicillin-resistant Staphylococcus aureus (EMRSA) 1, 3, 15 and 16. Mixed bacterial cultures of MSSA and EMRSA were incubated and subcultured after one, two and seven days. A proportion of MRSA in 50 randomly selected colonies was assessed. All strains of EMRSA (EMRSA 1, 3, 15 and 16) outgrew and virtually eradicated MSSA (NCTC 11561) after 24 h. The interaction between strains of PMSSA and the various strains of EMRSA was variable. PMSSA strain 1 was almost completely outgrown by EMRSA 1 and EMRSA 3 after seven days. Similarly there was a substantial increase of EMRSA 1 and 3 when tested against PMSSA strain 2. EMRSA 15 increased modestly against both strains, but EMRSA 16 failed to increase in proportion against either of the strains. We conclude that there is a complex interaction between various strains of EMRSA and MSSA. This interaction may have an important bearing on colonization of patients with MRSA.